Li recovery processes and onsite chemical production for li recovery processes

ABSTRACT

In this disclosure, a process of recycling acid, base and the salt reagents required in the Li recovery process is introduced. A membrane electrolysis cell which incorporates an oxygen depolarized cathode is implemented to generate the required chemicals onsite. The system can utilize a portion of the salar brine or other lithium-containing brine or solid waste to generate hydrochloric or sulfuric acid, sodium hydroxide and carbonate salts. Simultaneous generation of acid and base allows for taking advantage of both chemicals during the conventional Li recovery from brines and mineral rocks. The desalinated water can also be used for the washing steps on the recovery process or returned into the evaporation ponds. The method also can be used for the direct conversion of lithium salts to the high value LiOH product. The method does not produce any solid effluent which makes it easy-to-adopt for use in existing industrial Li recovery plants.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of U.S. application Ser. No.17/883,983, filed Aug. 9, 2022, which is a continuation application ofSer. No. 17/416,413, filed Jun. 18, 2021, which is the U.S. NationalPhase of International Application No. PCT/IB2019/001346, filed Dec. 19,2019, which claims priority to U.S. Provisional Application No.62/784,324, filed Dec. 21, 2018, and U.S. Provisional Application No.62/907,486, filed Sep. 27, 2019, the contents each of which areincorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

This invention relates generally to Li recovery processes and the onsiteproduction of chemicals used in Li recovery processes. The invention canbe used to improve Li extraction from various sources, including brinesources or lithium ore sources where hydrochloric acid, sodiumhydroxide, and/or sulfuric acid may be required. The invention caneliminate the need to outsource the acid and base feed materials byutilizing the already available brine onsite, whether derived from salarbrine or a brine solution that is produced in the lithium ore refiningprocess or a brine produced during a lithium ion battery recyclingprocess. The invention can also be used to directly convert lithiumcontaining salts, e.g. lithium chloride or lithium sulfate, to highervalue lithium hydroxide product. The invention can also provide a gasdiffusion electrode configured for use in a membrane electrolysis celland a method of producing the gas diffusion electrode. Additionally, theinvention can provide a membrane electrolysis cell for processing asalt-containing solution. The invention can also provide a process forpurifying or concentrating or producing LiOH using a membraneelectrolysis cell.

BACKGROUND OF THE INVENTION

Lithium has found an ever-increasing attention over the past few decadesdue to the advent of lithium ion batteries (LIBs) as the main source ofenergy storage for automobile and electronic application. The electricvehicle (EV) market, which is heavily depending on LIBs, is alsoexpanding at record pace over the last decade and is expected to share20% of the transportation market with internal combustion engines.Renewable energy generation including solar and wind applications arealso expected to rely on LIBs for load leveling purposes. Recovering,such as by recycling Li from LIBs, is considered to be a secondaryresource for lithium. Beside batteries, Li also has applications inglass and ceramics, chemicals and pharmaceuticals, metallurgicals andgreases. Accordingly, there remains a growing need for improved Lirecovery processes and related equipment.

SUMMARY OF THE INVENTION

Disclosed herein as an aspect of this invention is a gas diffusionelectrode for use in a membrane electrolysis cell. The gas diffusionelectrode comprises a diffusion layer configured to diffuse a gas; ahydrophilic catalyst layer disposed on a surface of the diffusion layer,the hydrophilic catalyst layer having a hydrophilicity greater than thatof the diffusion layer and being capable of transporting negative ions;and an ion exchange membrane disposed on a surface of the hydrophiliccatalyst layer, the ion exchange membrane being configured to exchangeions from the hydrophilic catalyst layer to an opposed surface of theion exchange membrane.

Also disclosed according to another aspect of this invention is a methodof producing a gas diffusion electrode for use in a membraneelectrolysis cell. The method comprises disposing a hydrophilic catalystlayer on a surface of a diffusion layer, the hydrophilic catalyst layerhaving a hydrophilicity greater than that of the diffusion layer; anddisposing an ion exchange membrane on a surface of the catalyst layer,the ion exchange membrane being configured to exchange ions from thecatalyst layer to an opposed surface of the ion exchange membrane and toprevent flooding of the catalyst layer.

A membrane electrolysis cell for processing a salt-containing solutionis also disclosed according to another aspect of this invention. Themembrane electrolysis cell comprises an inlet through which thesalt-containing solution is introduced into an interior of the membraneelectrolysis cell; an anode positioned to extend within the interior ofthe membrane electrolysis cell and positioned in an anode compartment; acathode comprising a gas diffusion electrode positioned to extend withinthe interior of the membrane electrolysis cell and positioned in acathode compartment, the gas diffusion electrode including a diffusionlayer configured to diffuse gas and a hydrophilic catalyst layerdisposed on a surface of the diffusion layer, the hydrophilic catalystlayer having a hydrophilicity greater than that of the diffusion layerand the hydrophilic catalyst layer being configured to transportnegative ions; a gas inlet through which a gas comprising O₂ isintroduced into contact with the gas diffusion electrode; a first ionexchange membrane interposed between the anode compartment and thehydrophilic catalyst layer of the gas diffusion electrode, the first ionexchange membrane being configured to exchange ions received from theanode to an opposed surface of the first ion exchange membrane; and atleast one outlet through which a product of the salt solution is removedfrom an interior of the membrane electrolysis cell.

Finally, a process for purifying or concentrating LiOH using a membraneelectrochemical cell is disclosed according to yet another aspect ofthis invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary process flow diagram of a Li recovery frombrine operation;

FIG. 2 shows an exemplary process flow diagram of a Li recovery fromrock mining operation;

FIG. 3 is a schematic diagram illustrating an exemplary membraneelectrolysis cell showing feed and product streams;

FIG. 4 shows the structure of the electrodes and membrane assemblyinside a first embodiment of a membrane electrolysis cell;

FIG. 5 shows the structure of the electrodes and membrane assemblyinside a second embodiment of a membrane electrolysis cell;

FIG. 6 shows the structure of the electrodes and membrane assemblyinside a third embodiment of a membrane electrolysis cell;

FIG. 7 shows the structure of the electrodes and membrane assemblyinside a fourth embodiment of a membrane electrolysis cell;

FIG. 8A shows an exemplary single layer gas diffusion electrode (GDE);

FIG. 8B shows details of the single layer gas diffusion electrode (GDE)of FIG. 8A;

FIGS. 9A and 9B show details of the contact interface between thecatalyst layer and the anion diffusion membrane of an exemplary bilayergas diffusion electrode (GDE);

FIG. 10 shows a first embodiment of a bilayer gas diffusion electrode(GDE);

FIG. 11 shows a second embodiment of a bilayer gas diffusion electrode(GDE);

FIG. 12 shows a schematic of functionality of the bilayer gas diffusionelectrode;

FIG. 13 shows a schematic of exemplary process steps to make the gasdiffusion electrode used as the oxygen depolarized electrode;

FIG. 14 shows a schematic of producing an embodiment of a catalystcoated membrane for use with an oxygen depolarized cathode;

FIGS. 15A and 15B show top and side cross sectional views, respectively,of an embodiment of a flow field on the cathode compartment;

FIG. 16 shows a process flow diagram for use of an embodiment of themembrane electrolysis cell as incorporated into a process for Lirecovery from brine;

FIG. 17 shows a process flow diagram for use of an embodiment of themembrane electrolysis cell as incorporated into a process for Liextraction from mineral rock;

FIG. 18 shows a process flow diagram for use of an embodiment of onsiteLiOH and Li₂CO₃ and HCl generation from LiCl brine as incorporated intoa Li recovery process from salar brines by the conversion ofLiCl→LiOH+HCl;

FIG. 19 shows a process flow diagram of an embodiment of onsite LiOH andLi₂CO₃ and HCl generation from LiCl brine as incorporated into arecovery process of Li from salar brines by the conversion ofLiCl→LiOH+HCl;

FIG. 20 shows a process flow diagram of an embodiment of onsitecrystallized LiOH generation from mixed LiCl and NaCl brine asincorporated into a recovery process of Li from salar brines by theconversion of LiCl→LiOH+HCl and by the conversion in the same cell ofNaCl→NaOH+HCl;

FIG. 21 shows a process flow diagram of an embodiment of onsite LiOH andH₂SO₄ generation from Li₂SO₄ as incorporated mid-stream into a processof lithium recovery in a hard rock mining operation by the conversion ofLi₂SO₄→LiOH+H₂SO₄;

FIG. 22 shows a process flow diagram of an embodiment of LiOH productionfrom Li₂CO₃, where HCl is produced and is recycled to dissolve theLi₂CO₃ in a closed-loop process;

FIG. 23 shows a process flow diagram of an embodiment of LiOH productionas incorporated into a recovery process of Li from a lithium brine bythe selective adsorption of Li using an ion exchange resin by theconversion of LiCl→LiOH+HCl such that the HCl is used to regenerate theion exchange resin;

FIG. 24 shows a process flow diagram of another embodiment of LiOHproduction as incorporated into a recovery process of Li from a lithiumbrine by the selective adsorption of Li using an ion exchange resin bythe conversion of LiCl→LiOH+HCl such that the HCl is used to regeneratethe ion exchange resin;

FIG. 25 shows a schematic of an embodiment of LiOH production fromLi₂CO₃;

FIG. 26 shows a current vs. time plot for a membrane electrolysis cellutilizing a single layer GDE with pure oxygen applied at the cathode;

FIG. 27 shows a current vs. time plot for a membrane electrolysis cellutilizing bilayer GDE as an ODC, with pure oxygen applied at thecathode; and

FIG. 28 shows a current vs. time plot for a membrane electrolysis cellutilizing bilayer GDE as an ODC, with air applied at the cathode.

DETAILED DESCRIPTION OF THE CERTAIN EMBODIMENTS OF THE INVENTION

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

The invention, according to one aspect, makes it possible to provide aunique gas diffusion electrode that can use ambient undried, humid airas a source of oxygen. This gas diffusion electrode, according to oneembodiment, is used only at the cathode of the membrane electrolysiscells as disclosed herein.

According to additional aspects of this invention, the invention makesit possible to improve Li extraction from various sources, includingbrine sources or lithium ore sources where hydrochloric acid, sodiumhydroxide, and/or sulfuric acid may be required; eliminate the need tooutsource the acid and base feed materials by utilizing the alreadyavailable brine onsite, whether derived from salar brine or a brinesolution that is produced in the lithium ore refining process or a brineproduced during a lithium ion battery recycling process; directlyconvert lithium containing salts, e.g. lithium chloride or lithiumsulfate, to higher value lithium hydroxide product; provide a gasdiffusion electrode configured for use in a membrane electrolysis celland a method of producing the gas diffusion electrode; provide amembrane electrolysis cell for processing a salt-containing solution;and/or provide a process for purifying or concentrating or producingLiOH using a membrane electrolysis cell.

As used herein the term, “hydrophobic” means lacking an affinity for,repelling, or failing to adsorb or absorb water. In particular, ahydrophobic substance is one which has a contact angle greater than 90°when a droplet of water is placed on it.

As used herein the term, “hydrophilic” means having an affinity for andbeing capable of adsorbing or absorbing water. In particular, ahydrophilic material is one where the contact angle between a droplet ofwater and the material is less than 90°.

As used herein, the terms “oxygen depolarized cathode” or “ODC,” and“gas diffusion cathode” or “GDC,” may be used interchangeably and/orrefer to the same structure, which is used for example, as the cathodein a membrane electrolysis cell.

Lithium Extraction from Brines

An increasing demand for the lithium requires matching productionlevels. In its free metallic form, lithium is extremely reactive. Hence,it is usually found in the form of mineral compounds comprising about0.007% of the earth's crust as the primary source of lithium. Naturalsources of lithium include igneous rocks, springs, sea and ocean water,as well as salar brines, which are underground reservoirs that containhigh concentrations of dissolved salts, such as lithium, potassium, andsodium. These are generally found below the surface of dried lakebeds,known as salars.

Most lithium products are from brine sources, and most lithium isrecovered from the salar brines or salt lake brines that occur in theEarth's crust and which contain lithium. The concentration of lithium insea water is about 0.17 mg/L. Geothermal water and oil-well brines arealso another source of recoverable lithium. Lithium salt-containingbrines that may be produced during lithium ion battery recyclingprocesses are also a source of recoverable lithium. Lithium recoveryfrom these brines can be less expensive than mining the lithium from themineral rocks.

While the following disclosure presents embodiments directed torecovering lithium from salar brines, or from lithium brines producedduring processes to recover lithium from lithium ores, lithium saltsolutions, also referred to herein as “brine,” may also be sourced fromlithium ion battery recycling processes.

The first attempt at commercial extraction of lithium from salt lake wasreported in 1936 at the Searles Lake in the US. An ever-increasinggrowth in the extraction of lithium from the salt lakes has occurredsince then. The brine concentration, pond accessibility and locality forsolar evaporation, ratio of alkaline earth and alkali metal to lithiumand the complexity of the chemistry are important factors whenconsidering a brine as a Li recovery source.

The recovery of lithium from brine on an industrial scale can commencewith a series of multiple stages of continuous solar pond evaporation ofwater from the brine. The lithium concentration of major brineextraction operations in the world ranges from 0.06 to 0.15 wt. % of theelement lithium. This concentration reaches about 6 wt. % at the end ofthe evaporation cycle when it is ready for lithium carbonate (Li₂CO₃)production, or the production of LiOH as the desired end product. Duringthe evaporation stage, salts with a lower solubility limit than thelithium salts precipitate out of the brine. Halite salts such as (NaCl)is the first to precipitate followed by sylvite (KCl), sylvanite(NaCl·KCl) and other salts. KCl is the main by-product in most brinerecovery operations.

Some salar brines which contain higher Mg, Ca, and B content, requireadditional processing steps to remove these elements. Magnesium andcalcium are preferably removed in order for the final product to beutilized in battery applications. Boron contamination is also harmful tothe quality of the end product and has to be removed during theextraction process to achieve LIB quality requirement. During theevaporation stage, some compounds of Mg such as carnallite(KCl·MgCl₂·6H₂O) and bischoffite (MgCl₂·6H₂O) precipitate at around 4.4wt. % elemental Li concentration in the brine. After further evaporationto reach 5-6 wt. % Li, the co-precipitation of compounds such as lithiumcarnallite (LiCl·MgCl₂·6H₂O) along with lithium carbonate or lithiumchloride results leaves the final product with Mg contamination. Toobtain a pure lithium carbonate or chloride product, these contaminantsare preferably removed.

The removal process of Ca, Mg and B from the brine involves solventextraction and precipitation stages. For example, boron can be removedby a solvent extraction process or the precipitation of boric acid. Inthe solvent extraction method, extractants such as high aliphaticalcohols or other organic solvents are used in an acidic environment toremove boron from the brine. Under acidic conditions, boron compoundsform boric acid which precipitates out of the brine.

Numerous systems have been reported on development of boron removalprocess from lithium-containing brine. For instance, U.S. Pat. No.3,855,392 describes the use of high aliphatic alcohol for extraction ofboron from magnesium brine. Similarly, U.S. Pat. No. 4,980,136 describesthe use of high aliphatic alcohol with 6 to 16 carbon atoms dissolved inkerosene at pH levels of 1 to 2 for extracting the boron from brine.U.S. Pat. Nos. 3,424,563; 5,939,038 describe the use of diols which areorganic molecules containing two OH groups in their structures asextractants for boron removal. U.S. Pat. No. 4,261,960 discloses aboron, magnesium and calcium removal process involving the use of slakedlime (Ca(OH)₂) and calcium chloride solution to precipitate calciumborate hydrate as well as magnesium hydroxide and calcium sulfatedihydrate from the brine.

Magnesium and calcium can be removed from the brine through severalprecipitation processes. Converting the dissolved magnesium salt intomagnesium hydroxide removes magnesium from the brine. Calcium hydroxide,lime (CaO) or slaked lime (Ca(OH)₂) is added to the brine to increasethe pH and form Mg(OH)₂ which has limited solubility in water. Duringthis process, sulfate ions also precipitate in the form of CaSO₄ andboron precipitates in the form of calcium borate hydrate. Theprecipitation of Mg(OH)₂ is also beneficial in further removal of boronthough surface adsorption mechanism.

The adjustment of pH can be effected through different mediums. U.S.Pat. Nos. 4,036,713 and 4,207,297, disclose using the lithium hydroxideproduct form the recovery product to raise the pH of the initial brineand thereby precipitating the magnesium hydroxide. These patents alsodisclose the use of other basic solutions including sodium hydroxide orcalcium hydroxide. U.S. Pat. No. 6,143,260 discloses the use of limedmother liquor from a previous lithium precipitation stage to remove Mgby precipitating the Mg(OH)₂. U.S. Pat. No. 6,048,507 discloses abicarbonation process comprising mixing the impure lithium carbonatebrine with CO₂ gas under pressure to precipitate unwanted species ofiron, magnesium and calcium. Further removal of Fe, Mg and Ca ions iscarried out by selective ion exchange process.

U.S. Pat. No. 5,993,759 describes a multistage process for purifying thelithium carbonate brine of high Mg content using soda ash, slaked limeand organic extractant. The process starts with an acidification stepusing hydrochloric acid (HCl) to maintain a pH level of 0 to 4 in orderto produce boric acid (H₃BO₃) and which is removed throughcrystallization. The resulting boron-depleted brine is further purifiedusing organic solvent extractant to achieve boron-free brine. Theresulting brine diluted by a mother liquor is then treated by additionof sodium carbonate (soda ash) yielding magnesium carbonate as the solidprecipitate. The rest of magnesium in the brine is further precipitatedusing calcium hydroxide and so forms Mg(OH)₂. During this stage, anycalcium introduced to the brine also precipitates in the form of calciumcarbonate due to the presence of sodium carbonate in the brine. Theaddition of soda ash at the last step results in the precipitation oflithium carbonate.

U.S. Pat. No. 8,691,169 discloses that the sequence of adding thesecompounds to the brine plays an important role in producing high puritylithium carbonate. The disclosure shows that the addition of calciumhydroxide before the brine removal step can remove all soluble magnesiumas well as some boron and surface ions in the post-evaporation brine.The rest of boron in the brine is removed via solvent extraction. Theresulting magnesium and boron-free brine is then carbonized by soda ashto obtain high purity lithium carbonate precipitates. The obtainedLi₂CO₃ was further purified by adding carbonic acid, forming lithiumbicarbonate and then precipitation of pure lithium carbonate is effectedby heating the bicarbonate-containing brine. The last stage is disclosedto remove all sodium and calcium impurities from the final precipitateproduct.

The final lithium product is produced from the resulting brine fromwhich most of Mg, B, Ca, and other species have been removed. Soda ash(sodium carbonate, i.e. Na₂CO₃) is a reagent for converting the soluteLi ions into Li₂CO₃ precipitate. This precipitate can be used intechnical or high purity grade. Depending on the nature of the process,the initial composition of the brine and the customer demand, the Lirecovery process can be designed and modified. For technical gradelithium carbonate, the common recovery process of pond evaporation andconcentration, selective removal of contaminants and carbonation issufficient.

For high purity and battery grade product, several extra processingstages have been proposed. For instance, U.S. Pat. No. 9,169,125 B2discloses the use of spray drying, washing and carbonation after thecontaminant removal to obtain purified lithium carbonate product.Bicarbonation of the lithium carbonate can be used for purifying thefinal product.

U.S. Pat. No. 8,691,169 B2 discloses the use of carbonic acid to formlithium bicarbonate from the lithium carbonate. The resultingbicarbonate is then decomposed to purified carbonate upon exposure toheat at 50 to 95° C. Another method for further purification of thebrine prior to lithium carbonate precipitation is the implementation ofion exchange resins. Lithium carbonate products with purity as high as99.9% can be achieved by using this method in the process.

WO Pat. Appln. Publ. No. 2013/036983 discloses the use of ion exchangeresins throughout the recovery process. Also disclosed therein is an ionexchange resin for boron removal, which replaces the solvent extractionmethod. An ion exchange resin to remove the trace soluble divalent andtrivalent species containing Mg, Ca and Fe from the brine during thefinal stages of the recovery is also disclosed. U.S. Pat. No. 8,641,992discloses ion exchange resin by which Mg ions are selectively removedfrom the brine.

Generally, a process for the recovery of lithium from brine can beillustrated as shown in FIG. 1 . The first stage of recovery consists ofmultiple pond evaporative concentration steps that remove high levels ofsodium and potassium salts for instance, NaCl and KCl and possiblyothers by precipitation, since these are less soluble than the desiredlithium salts. The evaporation thus increases the lithium concentrationin the brine. Some of the magnesium in the form of precipitated MgCl₂ isalso removed during this evaporative stage. Next stages involve theremoval of boron, calcium, magnesium as the main sources of impurity inthe brine. Removal of the B, Ca and Mg ions is carried out usingrepeated pH adjustment, solvent extraction and precipitation steps toensure maximum ion removal. Ion exchange removal of leftover tracemonovalent, divalent and trivalent ionic species other than the lithiumfurther purifies the brine. Introduction of soda ash, Na₂CO₃, to convertthe dissolved lithium salts to lithium carbonate, Li₂CO₃, is the lastmajor step to produce technical and high purity lithium carbonate.

As expected, almost every brine recovery operation, no matter the sourceof the brine heavily relies on the use of reagents throughout theprocess stages. Generally, more than 50% of the overall operating costsin lithium production usually stems from the cost of these reagents. Themajor reagents contributing to the recovery cost are soda ash (Na₂CO₃),lime (primarily CaCO₃) and slaked lime (Ca(OH)₂), caustic soda (NaOH),hydrochloric acid (HCl), extractants such as high aliphatic alcohols orother organic solvents, and sulfuric acid (H₂SO₄).

Soda ash or sodium carbonate (Na₂CO₃) is the major component of alithium recovery process due to its importance for various lithiumrecovery stages. It is used to remove calcium from the brine throughprecipitation of CaCO₃. It is also the sole reagent used for producinglithium carbonate.

Slaked lime or calcium hydroxide (Ca(OH)₂) produced from heating andthen hydrating the lime (primarily CaCO₃), can be used for the removalof magnesium and some sulfate ions. Different grades of lime could beused based in the use of lime in the initial evaporation pond or thelithium carbonate processing plant. For instance, the removal ofmagnesium chloride and sulfate and other sulfate ions from the brineusing the slaked lime occurs according to the following reactions:

MgCl₂(aq.)+Ca(OH)₂(aq.)⇔Mg(OH)₂(s)+CaCl₂)(aq.)  1)

MgSO₄(aq.)+Ca(OH)₂(aq.)⇔Mg(OH)₂(s)+CaSO₄(s)  2)

Na₂SO₄(aq.)+Ca(OH)₂(aq.)⇔CaSO₄(s)+NaOH(aq.)  3)

Caustic soda or sodium hydroxide (NaOH) is another chemical reagent thatcan be used in different stages of the processing plant. It can be usedas the stripping agent to regenerate the solvent after the boron removalusing the organic solvent extraction. It is also the most suitablealkali metal hydroxide for the removal of magnesium because it producesvery high purity Mg(OH)₂ as a by-product. Another important use of theNaOH is during the water treatment process for pH adjustment, as well asfor ion exchange resin regeneration.

Similar to caustic, hydrochloric acid (HCl) can be used as a reagent ina variety of the process steps throughout the recovery process. It canbe used as a pH modifier during the boron solvent extraction step sincethe initial stage of solvent extraction normally requires an acidicenvironment. Another use of hydrochloric acid is in the concentrationponds (also called evaporation ponds) where adding it prevents theunwanted precipitation of lithium carbonate. HCl is also the mainreagent for transformation of lithium carbonate into lithium chloridewhen needed. Hydrochloric acid is also used to regenerate the acidexchange resins which are used for selective ion removal from brine.Sulfuric acid (H₂SO₄) can be stored in the concentrated 98% form and canbe used for de-scaling and cleaning of the lithium carbonate processingplant.

The major constituents of the reagent costs are soda ash, contributingto almost 50%, lime with around 15%, caustic soda (NaOH) with about 7%and hydrochloric acid with 1% of the total reagents costs. In some caseswhere the ratio of Mg:Li is high, this contribution of the soda ash canbe as much as 80% of the total reagent costs. The on-site production ofthese chemicals can be very beneficial toward reducing operating costs.

Disclosed herein, as an aspect of this invention, is an electrodialysismulti-compartment system which allows for the on-site production ofcaustic soda (NaOH) and hydrochloric acid (HCL) from the existing brinein the evaporation ponds. The caustic soda is then transformed intosodium carbonate (soda ash) and sodium bicarbonate (NaHCO₃) using thereadily available carbon dioxide (CO₂) that is used on-site. The systemis fully controllable and allows for the desired concentration ofproduct. The capability to tune the concentration of NaOH and HClmitigates the operating costs associated with shipping and storingconcentrated solutions.

Lithium Extraction from Rock Mining

Several minerals contain Li in their structure. For example, at leastfour minerals have found interest as viable Li sources. These areLepidolite (K(Li,Al,Rb)₂(Al,Si)₄O₁₀(F,OH)₂), Spodumene (LiAl(SiO₃)₂),Petalite (LiAlSi₄O₁₀), and Amblygonite ((Li,Na)AlPO₄(F,OH)). Amongthese, Spodumene is usually the most important ore for commercial Liproduction. (Helvaci, C., 2003. Presence and distribution of lithium inborate deposits and some recent lake waters of West-Central Turkey. Int.Geol. Rev. 45 (2), 1-14.). Expected growth in demand over the comingcentury for lithium batteries used in power hybrid and fully electricautomobiles has raised interest in lithium production (Tahil, 2007,2008; Bradbury, 2008).

Various methods have been developed to obtain lithium from lithium ores(Victor, K. A., 1953. Method of recovering lithium compounds fromlithium minerals. U.S. Pat. No. 2,793,933; Walter, R., Bichowsky,Francis R., 1935. Method of recovering lithium from its ores. U.S. Pat.No. 2,020,854.; Robinson, G. P., 1961. Recovery of lithium from ore.U.S. Pat. No. 2,983,576; Moon, K. S., Fuerstenau, D. W., 2003. Surfacecrystal chemistry in selective flotation of spodumene (LiAl[SiO₃]₂) fromother aluminosilicates. Int. J. Miner. Process. 72 (1-4), 11-24; Saeki,S., Lee, J., Zhang, Q., Saito, F., 2004. Co-grinding LiCoO₂ with PVC andwater leaching of metal chlorides formed in ground product. Int. J.Miner. Process. 74 (Supplement 1), 5373-5378; Büyükburc, A., Maras, I.,Ioglu, D., Bilici, M. S. U., Köksal, G., 2006. Extraction of lithiumfrom boron clays by using natural and waste materials and statisticalmodelling to achieve cost reduction. Miner. Eng. 19 (5), 515-517;Jandová, J., Dvorák, P., Vu, H. N., 2010. Processing of zinnwalditewaste to obtain Li₂CO₃. Hydrometallurgy 103 (1-4), 12-18; Brandt, F.,Haus, R., 2010. New concepts for lithium minerals processing. Miner.Eng. 23 (8), 659-661; Chen, Y., Tian, Q., Chen, B., Shi, X., Liao, T.,2011. Preparation of lithium carbonate from spodumene by a sodiumcarbonate autoclave process. Hydrometallurgy 109 (1-2), 43-46). Lithiumcan be extracted from lepidolite using the sulfate acid and the limemethods (Distin, P. A., Phillips, C. V., 1982. The acid extraction oflithium from the granites of South West England. Hydrometallurgy 9 (1),1-14). However, the extraction of lithium by the sulfate acid methodoften uses high concentration acid and the purification procedure can becomplex. The lime process uses limestone and can require a large amountof energy.

Wadman and von Girsewalt (Ellestad, Reuben B., Clarke, Fremont F., 1955.Extraction of lithium from its ores. Min. Eng. 7, 1045) ground lithiumsilicate ore (Lepidolite (K(Li,Al,Rb)₂(Al,Si)₄O₁₀(F,OH)₂), Spodumene(LiAl(SiO₃)₂)) with an excess of alkali sulfate (usually K₂SO₄) in atleast a 1 to 1 proportion and heated the mixture to a relatively hightemperature. At elevated temperature ion exchange occurred forminglithium sulfate which, together with the excess potassium sulfate, wasdissolved by leaching with water. Successful operation of this processrequired thorough mixing and careful temperature control. In addition,high consumption of K₂SO₄ may add cost. A mixture of alkali sulfates andalkali oxide was used as reactant and the results demonstrated that anadvantageous effect was obtained when the lepidolite was roasted with amixture of Na₂SO₄, K₂SO₄ and CaO.

In order to process spodumene as described in U.S. Pat. No. 2,516,109,α-spodumene raw material is first converted to β-phase by roasting at1100-1300° C. A typical lithium extraction from spodumene mineral isshown in FIG. 2 . Tahil (Tahil, W., 2010. How much lithium does a Li ionEV battery really need? Meridian Int. Res., (Mar. 5, 2010).) reportedthe roasting of spodumene in a kiln at ˜100° C. The calcine was mixedwith sulfuric acid and roasted at 250° C. and subsequently leached inwater to yield an aqueous solution of lithium sulfate. Reaction ofβ-spodumene with H₂SO₄ is shown as reaction 4). (Mcketta, J. J., 1988.Lithium and lithium compounds. Encyclopedia of Chemical Processing andDesign vol. 28. Marcel Dekker)

Li₂O·Al₂O₃·4SiO₂(s)+H₂SO₄(conc.)→Li₂SO₄(s)+Al₂O₃·4SiO₂(s)  4)

Lithium carbonate can be recovered by the addition of sodium carbonateto the solution after pH adjustment, purification and evaporation, shownin reaction 5.

Li₂SO₄(aq.)+Na₂CO₃(s)→Li₂CO₃(s)+Na₂SO₄(aq.)  5)

The world's first continuous plant to convert spodumene concentrate tolithium carbonate by calcination, roasting of calcine with H₂SO₄ andsubsequent water leaching, was commissioned in 2012 by Galaxy Resourcesin China (Clarke, G. M., 2013. Lithium-ion batteries: raw materialconsiderations. Am. Inst. Chem. Eng. 44-52). One of the drawbacks of thesulfuric acid method to treat lepidolite, petalite and zinnwaldite isthe requirement of a high concentration of acid and complex purificationprocesses, whereas spodumene needs to be converted to the more leachableβ-phase at higher temperature.

Overall, the cost of reagents during the rock mining process alsocontributes to 45% of the overall operation costs. Implementing methodsto reduce such costs is the next big step in lithium recovery process.Due to the intense use of chemical reagents during these processes, themost logical approach is to recycle them after the process.Electrochemical techniques provide flexible solutions for recyclingchemicals.

Electrochemical Processes for Onsite Reagent Recovery

The use of membrane electrolysis cells (also referred to aselectrodialysis cells) have been successfully implemented as describedherein to generate chemicals from a brackish water or brine stream. Theprocess involves the use of a series of ion exchange membranes stackedin an order specific to the components of the brine stream beingprocessed as well as the desired outputs. The membranes are designed toallow specific charged ionic species permeate through. Cation exchangemembranes transfer cationic species while anion exchange membrane onlyallow anions transport through the membrane structure. Bipolar membranesare another type which split water molecules into the H+ and OH−components. The movement of ions is enabled by applying an externalvoltage using a cathode and anode electrode. Under applied voltage,anions travels toward the positively charged anode while cations traveltowards the negatively charged cathode. Through careful placement ofmembranes, desired chemicals such acids, bases, and salts can beproduced. During an electrolytic process with aqueous catholyte andanolyte, gaseous species such as H₂ and O₂ may be generated on theelectrodes, due to the electrolysis of water.

Electrochemical desalination techniques rely on the above-mentionedprinciples of membrane electrolysis. A stream of concentrated brine orsalt passes through the separation device. An electrical current (DC)applied between anode and cathode generates ionic species. The anionsfrom the input salt solution migrate through anion exchange membrane(s)and combine with H+ ions generated on the anode and produce acid.Cations from the input salt solution likewise migrate through cationexchange membrane(s) and combine with OH− ions generated on the cathodeand produce base.

An example of salt splitting process is disclosed in U.S. Pat. No.2,829,095, where a combination of anion and cation exchange membrane wasused to dissociate NaCl salt into Na+ and Cl− ions. Sodium ions werethen combined with OH− from the cathode and produced NaOH. On the otherhand, chloride ions combined with H+ generated on the anode and produceHCl. The overall voltage required to achieve the splitting include thepotential for water decomposition and potential drop across themembranes and electrolyte solutions.

Another example of this process is the conversion of brine (NaCl) intoNaOH and chlorine gas during the chlor-alkali process described in U.S.Pat. No. 4,217,186. During this process, the NaCl brine is fed into theanolyte compartment while water (or NaOH) is fed into the catholytecompartment. Upon the application of voltage, sodium ions migratethrough a cation exchange membrane toward the cathode where they combinewith OH− ions produced by the electrolytic splitting of water on thecathode which forms NaOH. Chlorine gas is evolved in the anodecompartment according to reaction 6) while hydrogen gas is produced onthe cathode.

4Cl—→2Cl₂+4e−E⁰=1.36V  6)

The chlor-alkali process can be modified to reduce the overall cellvoltage and thereby reduce energy consumption. With the purpose ofeliminating the H₂ generation on the cathode and reducing overall cellenergy consumption, an oxygen depolarized cathode (ODC) has shownreduction in the required cell voltage in the chlor-alkali process. Anexample of such an application is disclosed in U.S. Pat. No. 4,191,618.The depolarization of the cathode by oxygen gas results in the formationof only hydroxyl ions thereby preventing the formation of hydrogen gas.In a hydrogen evolving scenario, the cathodic reaction in thechlor-alkali process is as follows:

4H₂O+4e−→2H₂+4OH−E⁰=−0.83V  7)

Using the oxygen depolarized cathode, where O₂ gas is applied to thecathode, the cathodic reaction changes to a hydroxyl formation reactionaccording to the reaction:

2H₂O+O₂+4e−→4OH−E⁰=0.401V  8)

Overall, the use of ODC and the application of pure oxygen at thecathode in the chlor-alkali process means that the overall cell voltageis approximately 1V less out of 3.3V in chlor-alkali cell. In most casesof ODCs, pure O₂ is required to operate the cell.

As will be described in detail below, the novel gas diffusion electrodedisclosed herein, permits the operation of a five-compartment membraneelectrolysis cell with an air stream, rather than pure oxygen applied atthe cathode. In the instant case, the anode reaction is not the chlorineevolution because there is no chloride solution there. Instead oxygen isevolved in the water oxidation reaction:

2H₂O(l)→O₂(g)+4H+(aq.)+4e−  9)

WIPO Pat. Appln. Publ. No. WO 2015/149185 A1 describes a membraneelectrolysis cell assembled using a combination of anion and cationexchange membranes. The cell was utilized to convert carbon dioxide gasand a saline brine stream into carbonate salt, hydrochloric acid anddesalinated water.

The present disclosure is related to the in situ process of generatingreagent chemicals useful during the regular lithium extraction processesfrom salar brines or other lithium-containing brines such as thosearising from lithium ion battery recycling operations, and from rockminerals. The process describes an electrochemical method for convertingwaste chemical streams into valuable reagent chemicals required duringthe lithium extraction operation. The disclosed electrochemical methodinvolves a multi-compartment membrane electrolysis cell which may beincorporated into current lithium extraction processes from salar brineand from lithium-containing ore without disturbing the process flow ofeither type of recovery process. The disclosed process is unique in thatit allows the attachment of the unique membrane electrolysis cell feedand product streams onto the commonly practiced lithium extractionprocesses, as well as the particular design of the gas diffusionelectrode (GDE) which is used as the cathode and optionally may also beused as the anode of the cell.

In the following description of the embodiments of the invention, themembrane electrolysis cell will be described, and then the details ofthe gas diffusion electrode that is used as the oxygen depolarizedcathode will be discussed in detail. Finally, particular uses of themembrane electrolysis cell that comprises the GDE as the cathode in theproduction of lithium will be described.

Membrane Electrolysis Cell

A schematic diagram of feed and product streams for a generalizedinventive membrane electrolysis cell is illustrated in FIG. 3 .

A solution being treated, also referred to herein as the brine feed, isfed into the depletion (also referred to as the salt depletion chamberor compartment) chamber and cations and anions migrate from the solutionin the depletion chamber to adjacent product chambers (which may bereferred to herein also as acid build up compartment or base build upcompartment), thereby reducing the ion concentration of the solution.The solution may be any saltwater solution such as brine, seawater orwastewater, or any solution being treated to reduce the concentration ofions therein, for example industrial waste solutions from oil and gas,mining, forestry, lithium ion battery recycling processes, etc. Any typeof aqueous or non-aqueous stream consisting of ions or non-ionic speciesthat could be made into ions by the addition of other chemicals or byprocessing could potentially be utilized as the brine feed.

As alluded to above, in electrodialysis, an electric potential gradientmay be generated between an anode and cathode. In an aqueous setting,the anode and the cathode generally undergo the following half-cellreactions respectively:

H₂O(I)→2H+(aq.)+½O₂(g)+2e−(anodic reaction)  10)

2H+(aq.)+2e−→H₂(g)(cathodic reaction)  11)

The dialysis cell, also referred to as the membrane electrolysis cell ofthe disclosed embodiments includes a plurality of compartments, creatinga “stack” of compartments, and the walls of the compartments compriseion exchange barriers separating the chambers. Ion exchange barriers(also referred to herein as membranes) in the membrane electrolysis cellgenerally do not require regeneration, thereby reducing the need forchemical inputs over ion exchange processes. Inorganic scaling of ionexchange barriers and ion exchange barrier fouling can be managedthrough polarity reversal, periodic flushes and/or acid washes, asnecessary.

The ion exchange barriers of the membrane electrolysis cell includecation exchange barriers which selectively allow migration of cations,and anion exchange barriers which selectively allow migration of anions.The ion exchange barriers may be water permeable. The ion exchangebarriers may be ion exchange membranes and may include, but are notlimited to, commercially available bi-polar membranes and membranes withchemical modifications. Non-limiting examples of such modifications are:(i) perfluorinated films with fixed pyridine or sulfonic groups; (ii)polyetherketones; (iii) polysulfonones; (iv) polyphenylene oxides; (v)polystyrene; (vi) styrene-divinyl benzene; (vii) polystyrene/acrylicbased fabrics with sulfonate and quaternary ammonium cations; (viii)polyfluorinated sulfuric acid polymers; or (ix)resin-polyvinylidenedifluoride fabrics. In alternative embodiments,other ion exchange barriers such as are known in the art may beutilized.

The membrane electrolysis cells (also referred to as electrodialysiscells) of the described embodiments generally include a cathode andanode, which may be constructed of conductive porous or non-poroussubstrates, and coated with a catalyst or catalysts. The ion exchangebarrier (such as an ion exchange membrane) may alternatively oradditionally be coated with a catalyst or catalyst. These catalysts mayenhance the rate of reactions in the electrolysis cell. Suitablecatalysts include, but are not limited to, precious or non-precioustransition metals and their compounds (e.g. oxides, nitrides, etc.). Thecatalysts could be supported onto for example metal, metal oxides, metalnitrides, etc. or unsupported. A mixture of one or more catalysts,optional binder and other optional additives (for example hydrophilicand/or hydrophobic additives to control liquid and gas bubble removal),may be applied to the either or both of the cathode and anode electrodesand/or ion exchange barrier by a variety of techniques known in the art,such as spraying, sputtering, screen printing and the like. Fluids canflow in the cell via flow fields (open channels like serpentine,inter-digitated, etc.), porous closed channels, or open pocket. The cellcould be operated under pressure or pressure differentials.

In operation, an electric potential may be applied between the cathodeand anode to facilitate the occurrence of electrochemical reactions atthe electrodes and migration of ions across the ion exchange membranes.In a membrane electrolysis cell (also referred to as a dialysis cell),an electric potential may be applied between conductors to create anelectric field to enhance migration of ions across the ion exchangemembranes and chambers without any electrochemical reactions occurring.However, the application of an electric potential between the conductorsis not necessary for operation as ions may diffuse through the ionexchange membranes under the influence of other transport mechanismssuch as a concentration gradient.

In the described embodiments of the membrane electrolysis cell,solutions may be conveyed into and away from chambers of the membraneelectrolysis cell using a manifolding assembly which may includeconduits, optional valves and other equipment known in the art to conveysolutions to and away from chambers of a membrane electrolysis cell.

As the FIG. 3 schematic shows, a feed brine stream is supplied to thecell. Other inputs include air and/or oxygen and/or carbon dioxideand/or hydrogen gas or a mixture of these as well as electrical power.The product streams may include the desalinated water, acid, base and/orcarbonate salts. The acid materials may be hydrochloric acid (HCl) orsulfuric acid (H₂SO₄) depending on the composition of the feed brine.The base product may be sodium hydroxide (NaOH), potassium hydroxide(KOH) or lithium hydroxide (LiOH) or a mixture, depending on the natureof the feed brine. Carbonate and bicarbonate salts may also be producedupon the addition of carbon dioxide to the gas feed stream. These saltsmay also be produced separately by sparging carbon dioxide into thesodium hydroxide product in a later stage.

A first embodiment membrane electrolysis single cell consists of fivecompartments as depicted in FIG. 4 . The cell contains an oxygendepolarized cathode (ODC) (described in more detail later) in a cathodecompartment, a dimensionally stable anode (DSA) in an anode compartment,as well as two anion and two cation exchange membranes stacked inalternating fashion, so as to define the compartments of the cell.

The electrochemical processes involved in the first membraneelectrolysis cell are cathodic reaction on an ODC, anodic reaction on aDSA, acid formation in the acid build up compartment, base formation inthe base build up compartment and salt splitting in the salt depletioncompartment. The use of a first anion exchange membrane near the cathodecompartment allows for transport of the hydroxyl ions generated by thecathode into the base compartment. Another important use of thismembrane is to avoid the flooding of gas diffusion electrode cathode(i.e., the ODC) in contact with the base solution. A first cationexchange membrane between the salt depletion compartment and the basebuild up compartment enables the transport of salt cations (Na+ in thecase of NaCl or Na₂SO₄ as the feed brine or Li in the case of LiCl orLi₂SO₄) from the salt depletion compartment into the base build upcompartment. The combination of sodium ions with hydroxyl ions resultsin the formation of sodium hydroxide in the base build up compartment.The anodic reaction results in generation of protons (H+) which are thentransported through a second cation exchange membrane into the acidbuild up compartment. The protons then combine with anions transportedthrough a second anion exchange membrane from the salt depletioncompartment to form acid. Depending on the nature of salt, anions (Cl−in the case of NaCl and SO₄ ²⁻ (sulfate) in the case of Na₂SO₄),hydrochloric acid (HCl) or sulfuric (H₂SO₄) acid is formed. Further, ifLiCl is fed to the salt depletion compartment, HCl will build up in theacid build up compartment and LiOH will be formed in the base build upcompartment. Finally, if Li₂SO₄ is utilized as the brine feed stream,sulfuric (H₂SO₄) acid and LiOH will be formed in the acid build upcompartment and the base build up compartment, respectively.

A second embodiment of the membrane electrolysis cell is shown in FIG. 5. In this second embodiment, the membrane electrolysis cell comprisesfour compartments. In this embodiment, the cathode, which is an oxygendepolarized cathode, like the first embodiment, is housed in a cathodecompartment, which is defined by a first anion exchange membrane. Thecathode compartment is in fluid communication with a base build upcompartment via the first anion exchange membrane. The base build upcompartment defined by the first anion exchange membrane and a cationexchange membrane. As can be seen in FIG. 5 , the base chamber thus influid communication with a salt compartment, which may also be referredto as the salt depletion compartment, via the cation exchange membrane.The second anion exchange membrane defines an anode compartment, whichhouses the anode. The anode therefore is in fluid communication with thesalt chamber via the second anion exchange membrane.

In the membrane electrolysis cell second embodiment shown in FIG. 5 , anexemplary feed brine comprising aqueous NaCl may be fed to the salt (orsalt depletion) compartment. Oxygen, which may be in the form of air,and preferably is humidified air, produced by bubbling the air streamthrough water is fed to the oxygen depolarized cathode. The oxygendepolarized cathode may be a bilayer gas diffusion electrode (describedin detail below). When a voltage is applied across the anode andcathode, the positive Na+ migrate towards the negatively charged cathodecompartment through the cation exchange membrane and remain in the basebuild up compartment, since they cannot pass through the first anionexchange membrane. Likewise, the OH− anions produced at the ODC build upin the base build up chamber since they will migrate away from thenegatively charged cathode through the anion exchange membrane andtowards the positively charged anode. Like the Na+ ions, the OH− ionsremain in the base build up chamber because they cannot pass through thecation exchange membrane. Therefore, NaOH is formed in the base build upchamber. As shown in FIG. 5 , the Cl− anions migrate away from the saltcompartment to the anode via another anion exchange membrane. The Cl−anions combine at the anode to form Cl₂, i.e. chlorine gas. Note thatbecause the membrane electrolysis cell is in an aqueous environment,some water oxidation reaction takes place at the anode compartment,since the anode compartment and the salt (also called salt depletion)compartments are separated by an anion exchange membrane.

Importantly, a person having skill in the art can appreciate that ifLiCl in an aqueous solution is used as the brine feed stream rather thanthe exemplary NaCl, LiOH will be produced in the base build upcompartment. In the case of LiCl, chlorine gas will still be produced atthe anode. Analogous to the first embodiment membrane electrolysis cell,Na₂SO₄ or Li₂SO₄ can also be used as the brine feed streams in thesecond embodiment membrane electrolysis cell, and will therefore produceNaOH and LiOH, respectively in the base build up compartment and willproduce H₂SO₄ at the anode.

A third embodiment membrane electrolysis cell is shown in FIG. 6 andcomprises three compartments. As shown in the figure, the anode ishoused in the salt depletion chamber, this third embodiment comprisestwo other compartments; a cathode chamber and a base build up chamber.Thus, the brine feed essentially is fed onto the anode. As in the firstand second embodiments, the cathode comprises a bilayer oxygen depletioncathode, to which is fed oxygen, preferably in the form of air, and morepreferably in the form of humidified air.

Therefore, as shown in FIG. 6 , OH− ions are evolved at the cathode andthey migrate away from the negatively charged cathode, through an anionexchange membrane which defines the cathode compartment and into thebase build up compartment. For an exemplary feed brine of aqueous NaCl,as shown in FIG. 6 , the Na+ ions formed in the anode/salt depletioncompartment migrate away from the positively charged anode, through acation exchange membrane, which defines the anode/salt depletioncompartment, and into the base build up chamber. NaOH is thereforeformed in the base build up chamber.

As shown in FIG. 6 , chlorine gas is evolved at the anode, since Cl−anions are formed at the anode. Like the first and second embodiments ofthe membrane electrolysis cell, if the feed brine comprises an aqueoussolution of LiCl, LiOH will form in the base build up compartment andCl₂ will evolve at the anode. If aqueous Na₂SO₄ is the feed brine, NaOHwill be formed in the base compartment and H₂SO₄ will form at the anode.Finally if aqueous Li₂SO₄ is the feed brine, LiOH will be formed in thebase compartment and H₂SO₄ will form at the anode.

A fourth embodiment membrane electrolysis cell is shown in FIG. 7 . Inthis embodiment, there is a single ion exchange membrane which may be acation exchange membrane. Like all of the other embodiments, the cathodeis an ODC that uses O₂, but air may be used as a source of O₂. As in theprevious embodiments, an aqueous NaCl solution is shown as an exemplaryfeed brine, but a person having skill in the art can comprehend thatsimilar transport of the ions will occur as for NaCl, depending on theionic species in the feed brine. In this fourth embodiment a brine feedcomprising NaCl is fed to the anode compartment. The Na+ ions thus movethrough the cation exchange membrane into the cathode compartment. Sincethe cathode comprises an ODC, OH− ions are formed at the cathode andthere base, i.e. NaOH is formed in the cathode compartment. In thisembodiment, it is clear that the cathode compartment and the basecompartment are the same. As shown in FIG. 4 , the Cl− ions combine toform chlorine gas at the anode.

Any of the membrane electrolysis cells described herein may optionallyincorporate any of the following of features:

-   -   Flow fields to improve oxygen and water transport to and away        from the membrane electrolysis cell;    -   Designed ion exchange membrane properties and feedback loops can        be incorporated to control the concentrations of the produced        acid and base.

Gas Diffusion Electrodes

As mentioned briefly above, an important component of the membraneelectrolysis cells disclosed herein is the unique gas diffusionelectrode that is used as the oxygen depolarized cathode. This gasdiffusion electrode allows the membrane electrolysis cell to operateusing air as the oxygen source at the cathode. This is a significanteconomic and safety advance in the ability to incorporate these cellsinto lithium recovery processes.

A gas diffusion electrode is shown schematically in FIGS. 8A and 8B.Typically, in these gas diffusion electrodes that can be used as theoxygen depolarized cathode, a catalyst is deposited directly on thesurface of the gas diffusion layer (GDL) as shown in FIG. 8A. Generally,the catalyst is either hydrophobic or hydrophilic. As shown in FIG. 8B,indicated by the spread-out water droplet on the catalyst surface, thecatalyst is hydrophilic. It should be understood that in FIG. 8B as wellas subsequent FIGS., that a water droplet, whether spread out toindicate a hydrophilic surface, or shown as sitting on top of thesurface, to indicate that the surface is hydrophobic, do not indicatethat water is actually residing on said surface. The water droplets aremerely a convenient way to indicate whether the surface shown ishydrophilic or hydrophobic.

As shown in FIGS. 8A and 8B, in the single layer GDE or ODC, the layercomprises a catalyst on the gas diffusion layer, optionally with ananion exchange membrane disposed on the opposite side of the GDL (notshown) from the catalyst.

The implementation of the ODC in the membrane electrolysis cell requiressignificant changes to cell design. A porous gas diffusion electrode(GDE) is usually used as the cathode electrode. This is because of therequirement of a three-phase boundary where the three reactants (oxygengas, liquid water and electrons) must be present all at the same time.The most crucial factor is the ease of oxygen gas access to the activearea where the reaction takes place.

It should be understood that the terms, “upper,” middle,” “lower,” etc.in the following discussion relate only to the relative placement of thelayers in the FIG. under discussion and are not necessarily applicableto the structure while it is in use.

A key feature of the inventive gas diffusion electrode (GDE), alsoreferred to herein as an oxygen depolarized cathode (ODC) or a gasdiffusion cathode (GDC) is that the catalyst layer is deposited directlyon the anion exchange membrane, as shown in FIG. 9A as a cross section.This catalyst coated membrane (CCM) structure allows for better ion (OH−or other anions in the gas stream) transport through the contactinterface between the catalyst and membrane. It should be understoodthat the catalyst coated membrane refers to the anion exchange membraneat the interface between the cathode and base compartment which iscoated with a catalyst layer at one side. Importantly, this gasdiffusion electrode that has the hydrophilic catalyst layer applieddirectly under the anion exchange membrane is used only on the cathodeside in all of the membrane electrolysis cells disclosed herein.

FIG. 9B shows a cross sectional view of the bilayer ODC showing how thecatalyst layer is in direct contact with a porous diffusion layer. Thecatalyst layer may be hydrophilic or hydrophobic depending on theoperation. Looking closely at FIG. 9B, the uppermost layer in FIG. 9B isthe anion exchange membrane. In a membrane electrolysis cell asdescribed herein, the anion exchange membrane will be facing the basebuild up chamber, in the case of the first embodiment (five compartment)membrane electrolysis cell described above. Directly below the anionexchange membrane is a hydrophilic catalyst layer. This hydrophiliccatalyst layer is disposed directly on the first gas diffusion layer(GDL). As shown in FIG. 9B, below the first gas diffusion layer is anoptional second gas diffusion layer. These gas diffusion layers areknown in the art and are hydrophobic.

The gas diffusion layer may be made of for example, carbon fiber paper,carbon felt, carbon cloth, porous metal structures, or other porousmaterials which can conduct electrons and provide ability for a gas todiffuse. The reduction reaction of water to produce hydrogen can occuron many different materials depending on the voltage applied to theelectrode. Without wishing to be bound by theory, the gas diffusionelectrode as disclosed herein may ensure that any hydrogen that isproduced due to reduction of water may react with oxygen to producewater. This water is then available to take part in the reduction ofoxygen at the cathode to produce OH−.

Finally, disposed directly on the second gas diffusion layer is anoptional hydrophobic catalyst layer. Importantly, if this optionalsecond catalyst layer is present, it is a hydrophobic catalyst layer. Ifthe optional second gas diffusion layer is not present, the optionalhydrophobic catalyst layer may be disposed directly on the opposite sideof the first gas diffusion layer from the required hydrophilic catalystlayer. Both the hydrophobic catalyst layer and the hydrophilic catalystlayer comprise a commercially available platinum/carbon powder, as isknown in the art. The Pt/C catalyst, also referred to as anelectrocatalyst, is mixed with an anion conducting ionomer. The additionof an anion conducting ionomer allows for better transport of OH− ionsfrom the reaction site toward the catalyst layer/membrane interface. Italso serves as the binder for the platinum/carbon powder or anotherelectrocatalyst. The binders may be polymers that have both hydrophilicand hydrophobic nature. The binders may be polymers that are exclusivelyhydrophobic or exclusively hydrophilic. For example, Nafion® (DuPont)ionomer is hydrophobic with hydrophilic pores while Teflon® (DuPont) maybe used as a binder as well but is only hydrophobic. The specific natureof the binder should allow gas diffusion, electrical conductivity andionic conductivity. In most cases, this is a balance of the amount ofbinder and catalyst. If there is too much polymer, the electrons willnot be conducted but if there is too little binder, the catalyst layerwill not be stable. The catalyst layer which comprises this mixture ishydrophilic. The anion exchange ionomer may be an ionomer, i.e, apolymer having some amount of ionizable copolymerized monomers. Theionomer may be a dispersed solution in a liquid which is mixed with thecatalyst and applied on the gas diffusion layer. An exemplary method ofcreating the ionomer binder may be to provide the ion exchange membraneis a solvent. This solution may be combined with the catalyst andapplied to the gas diffusion layer to produce the catalyst layer andthereby produce the gas diffusion electrode. Some ion exchange membranesmay be available as ionomers and do not need to be dissolved. Some suchanion exchange membranes are available commercially. Non-limitingexamples of such materials are the Fumion® FAA-3 ionomer from FuMA-Tech,which comprises a polyaromatic polymer, quaternary ammonium group(s) andhas bromide (Br−) as the counterion, or the anion exchange ionomer fromIonomr (Vancouver, BC, Canada) are suitable, for example.

The hydrophilic/hydrophobic character of the gas diffusion electrodeused as the oxygen depolarized cathode as described herein is shownschematically in FIGS. 10 and 11 . The hydrophilic or hydrophobic natureof the catalyst layers are indicated by the water droplets shown on theschematic diagrams. The anion exchange membrane is not shown in FIGS. 10and 11 . FIG. 10 shows an embodiment of the gas diffusion electrodewherein the optional hydrophobic catalyst layer is not present. However,since the gas diffusion layer itself is hydrophobic, the water dropletsitting on the gas diffusion layer indicates that the layer ishydrophobic. The non-optional hydrophilic catalyst layer which isdisposed directly on the gas diffusion layer and under the anionexchange membrane (not shown) is shown and is reported as hydrophilic bythe partially adsorbed water droplet in FIG. 10 . FIG. 11 is similar toFIG. 10 , but does show the optional hydrophobic catalyst layer,indicated by the water droplet that is not adsorbed on the catalystlayer. Like FIG. 10 , the anion exchange membrane is not shown, but isunderstood to be present and disposed on the surface of the hydrophiliccatalyst layer opposite the side disposed on the gas diffusion layer.

The benefits of the bilayer GDE (i.e., having the hydrophilic catalystlayer deposited directly under the anion exchange membrane and on top ofthe gas diffusion layer) stem from two distinct functionalities of itsstructure. First, the unique structure promotes the reaction of oxygenreduction to hydroxyl ions, reaction 12) rather than the undesirablehydrogen gas formation through reaction 13).

O₂+2H₂O+4e−→4OH−  12)

2H₂O+2e−→2OH−+H₂  13)

Hydrogen gas formation is undesirable because: (a) there is about 1V forthe overall cell voltage (i.e. extra energy consumption) when reaction13) occurs and (b) the hydrogen gas will make the operation of suchcells unsafe due to the presence of oxygen gas, hydrogen gas andplatinum catalyst at the same time in the cathode compartment.

Second, the specific configuration of hydrophilic and hydrophobic layersin this bilayer gas diffusion electrode allows for two simultaneousprocesses. First, and importantly, the hydrophobic diffusion layersupport and the optional hydrophobic bottom catalyst layer (if present)ensure that the humidified gaseous stream (oxygen or air) has easyaccess to the hydrophilic catalyst layer facing the anion exchangemembrane at all times which results in greatly improved mass transport,increasing the output capacity of the membrane electrolysis cell. Thehydrophobic catalyst layer (if present) at the bottom of the bilayer gasdiffusion electrode also ensures that any possible hydrogen gas formedat the hydrophilic catalyst layer facing the anion exchange membrane isimmediately combined with the incoming oxygen to form water. Thehydrophobic nature improves the reaction by pushing the water away fromthe electrode and preventing the catalyst sites from becoming flooded.If the catalyst sites are flooded with water, the oxygen will not beable to reach the cathode and the desired oxygen reduction reaction withwater to produce anions will not occur. This is shown schematically inFIG. 12 . Therefore the hydrophilic catalyst layer facing the anionexchange membrane provides for optimal cell performance and reducedcontact resistance, i.e. better ionic contact, while the other(optional) catalyst layer of the gas diffusion electrode must behydrophobic, (if present) to make sure that any water that may come inwith the air or through the anion exchange membrane or is formed throughthe chemical reaction of oxygen and hydrogen at the Pt catalyst isreacted with oxygen in the gas diffusion layer of the cathode. Thus,hydrogen gas will contact the platinum in the hydrophobic catalyst layeron the bottom of the gas diffusion electrode as it exits the cell. Theplatinum catalyzes the reaction of the hydrogen and oxygen to waterallowing the water to either react with oxygen electrochemically throughthe oxygen reduction reaction to produce hydroxide ions or to exit thesystem as water without reacting.

FIGS. 13 and 14 demonstrate a process for producing the bilayer gasdiffusion electrode used as the oxygen depolarized cathode. As shown inFIG. 13 , the process starts with two separate layers of the hydrophobicgas diffusion layer. In the following discussion, these gas diffusionlayers will be called “top” and “bottom” which refers only to theirrelative positions FIG. 13 .

The top gas diffusion layer is coated with a catalyst ink to provide thehydrophilic catalyst layer. The ink is a mixture of Pt/C catalyst powderand an anion exchange ionomer (serves as a binder and anion transferringagent) as described above. The bottom gas diffusion layer is likewisecoated with a catalyst ink containing a hydrophobic agent, e.g. apolytetrafluoroethylene (PTFE) dispersion. The ink is a therefore amixture of Pt/C catalyst powder, an anion exchange ionomer, whichpermits that catalyst layer to transport anion and, a PTFE dispersion(Teflon® is an example of such a material), which serves to render thecatalyst layer hydrophobic. Next, the two coated gas diffusion layersare placed together such that the uncoated sides are facing each other.The anion exchange membrane is disposed on the hydrophilic catalystlayer. In operation, it should be understood that completed oxygendepolarization cathode is oriented such that the hydrophilic catalystlayer is facing the anion exchange membrane which may face the basebuild up compartment, and hydrophobic catalyst layer is therefore facingthe incoming gas stream, which comprises O₂ and preferably is air andmore preferably is humid air. Regarding the process as shown in FIG. 13, it is clear that instead of two separate gas diffusion layers, eachcoated separately and then placed together with coated sides to theexterior, another embodiment of the process would be to coat each sideof a single gas diffusion layer.

FIG. 14 shows an embodiment of a catalyst coated membrane that could beused with a cathode, and would perform in a similar manner to thebilayer ODC described above. As shown schematically in FIG. 14 , thisembodiment comprises only the critical elements of the ODC. These arethe Pt/C catalyst combined with the anion exchange ionomer and thehydrophobic agent as the catalyst ink that is coated onto the alkalinestable anion exchange membrane. In this way, it would be possible toflow O₂ through the catalyst layer, while the anion exchange membranefaces the base build up chamber. The main advantage of the catalystcoated membrane is the improved contact resistance at the interfacebetween the catalyst layer and membrane. In terms of functionality, itperforms in similar manner as to the bilayer GDE or ODC.

The oxygen diffusion cathode as described above may optionally comprisea flow channel at the back side of the bilayer ODC. The flow channelserves two purposes. The first is that it provides improved distributionof the gas (air or oxygen). The flow channel can be serpentine, multipleserpentine, parallel flow, or interdigitated. Secondly, the “landing”parts of the flow field also act as pressure points, thereby providinguniform and sufficient contact through the overall cell. An example of aserpentine flow field in cathode compartment is shown in FIG. 15A as atop view. FIG. 15B shows a cross sectional side view of the channelscreated by such a serpentine flow channel. The dark squares are thecontact points of the flow field (i.e. the gas) and the gas diffusionelectrode/oxygen depolarized cathode.

Thus, the inventive bilayer gas diffusion electrode which preferably isused as the oxygen depolarized cathode in all of the embodiments of themembrane electrolysis cell described here has as part of its inventivefeature two layers, unlike the single layer gas diffusion electrode. Itis this unique geometry that allows these ODC's to use air, rather thanpure oxygen. In addition, the air may be humid or humidified, i.e.,ambient undried air may be fed directly to the gas diffusion electrode.The ODC also may use waste gas streams comprising oxygen as the oxygensource, for example an enriched oxygen stream resulting from a nitrogenproduction process.

Regarding all embodiments of the ODC, it may have the followingattributes:

-   -   Porous to allow for gas diffusion therein;    -   Ability to use humidified or humid undried ambient air as the        oxygen source;    -   Ability to use waste gas streams comprising oxygen as the oxygen        source;    -   Electrically conductive to allow for electrons to move;    -   Ionically conductive to allow the OH− product to diffuse out;    -   It may comprise a catalyst that can catalyze the oxygen        reaction, which is shown as reaction 8).    -   Hydrophobicity, i.e. the membranes may be hydrophobic;    -   Hydrophilicity; the membranes may also be hydrophilic;    -   The electrodes may comprise bilayer catalysts, i.e. the        individual catalyst layers themselves may have more than one        layer and each layer may comprise a different catalyst to        enhance their ion transport ability;

Regarding the dimensionally stable anode (DSA) it may have the followingattributes:

-   -   May also be a gas diffusion electrode to allow for the optional        use of H₂ gas at the anode.    -   Electrically conductive to allow for electrons to move;    -   Ionically conductive;    -   It may comprise an optional catalyst to catalyze the reactions        at the anode. If hydrogen gas is applied to the anode, the        reaction is shown as reaction 14).

H₂→2H++2e−  14)

Incorporation of Membrane Electrolysis Cell Comprising Gas DiffusionElectrodes into Lithium Recovery Processes

A salar brine lithium recovery process which utilizes the membraneelectrolysis cell to is shown in FIG. 16 . As can be seen, the sodiumchloride salt extracted from the evaporation stage in lithium brinerecovery process is fed into the membrane electrolysis cell as the feedbrine. The feed brine then undergoes a depletion process as it passesthrough the cell. The depletion is a result of the Na+ and OH− ionsmigrating out of the salt depletion compartment into the base build upand the acid build up compartments, respectively.

The ions that migrate out of the salt depletion chamber depend on whationic species are in the feed brine that is fed to the membraneelectrolysis cell. Hence, the desalinated water removed from the saltdepletion compartment can be re-concentrated and therefore recycled asfeed brine using the readily available salt (NaCl) stockpile from thesalts that are precipitated from the salar lake evaporation ponds. Theconcentration of feed brine plays an important role in providing themass transfer of the feed brine as well as supplying ions for acid andbase generation. Although the membrane cell can be operated at feedbrine concentrations as low as 0.1 wt. % of salt, it is beneficial tooperate at the maximum available feed brine concentrations.

The concentration of product acid and base that are removed from theacid and base build up chambers, respectively, may be adjusted accordingto the requirements of each particular process. Caustic soda (NaOH)concentrations in the range of 5-20 wt. % may be achieved using themembrane electrolysis cell disclosed herein. As shown in FIG. 16 ,typical uses of the NaOH produced in the membrane cell in a salar brinelithium recovery operation include, but are not necessarily limited to:

-   -   Neutralization and pH adjustment after the solvent extraction        process to recycle the solvent;    -   Provide alkalinity for precipitation and hardness removal;    -   Regeneration of the ion exchange resins used for hardness and        metal removal;    -   Conversion of lithium carbonate into lithium hydroxide through        caustization process.

All of the above-mentioned processes require caustic (NaOH)concentrations in the range of 5-20 wt. % which is within the rangeachievable by the membrane electrolysis cell.

Non-limiting examples of typical uses of the hydrochloric acid that canbe produced by the membrane electrolysis cell during a lithium brinerecovery process is as follows:

-   -   Adjustment of pH to remove boron during the solvent extraction        process;    -   Regeneration of the ion exchange resins used for hardness and        metal removal;    -   Conversion of lithium carbonate into lithium chloride.

A concentration of hydrochloric acid for the above-mentionedapplications is in the range of 4-12 wt. % which is achievable by themembrane electrolysis cell.

A lithium rock mining operation process which incorporates the membraneelectrolysis cell is shown in FIG. 17 . As can be seen, the sodiumsulfate (Na₂SO₄) salt which is the largest by-product of a lithium rockmining operation can be used as a feed brine to the membraneelectrolysis cell. As shown in FIG. 17 , feeding Na₂SO₄ to the membraneelectrolysis cell will produce NaOH and H₂SO₄ as the base and acid,respectively. These can be recycled and used as reagents in the lithiumrecovery process.

Sulfuric acid (H₂SO₄) is the main reagent necessary for extracting thelithium from the ore during the acid roasting process. As shown in FIG.17 , this chemical can be regenerated from the readily available sodiumsulfate by-product of the lithium ore hard rock mining operation.

Likewise, the sodium hydroxide that is produced can be used in a varietyof ways in the overall lithium production process. Non-limiting examplesof uses of sodium hydroxide during a lithium rock mining operation areas follows:

-   -   Provide alkalinity for precipitation and hardness removal;    -   Regeneration of the ion exchange resins used for hardness and        metal removal;    -   Conversion of lithium sulfate into lithium hydroxide by adding        NaOH to the process.

All above mentioned processes require NaOH concentrations in the rangeof 5-20 wt. % which is within the range achievable by the membraneelectrolysis cell.

FIG. 18 shows a first embodiment of onsite LiOH and HCl generation fromLiCl using the unique membrane electrolysis cell as disclosed herein. Inthe embodiment shown in FIG. 14 , the membrane electrolysis cell is usedto convert LiCl into LiOH in a lithium recovery from a salar brineprocess. As shown in FIG. 18 , the brine feed to the membraneelectrolysis cell is an solution of LiCl, which is fed to the saltdepletion chamber of the membrane electrolysis cell, shown in FIGS. 3-7. Oxygen gas, which is preferably in the form of air is fed to thecathode. The oxygen or air may optionally be humidified, e.g., bybubbling the gas through water before feeding it to cathode. The air mayoptionally be purified. As shown, the outputs from the cell are LiOH,which is removed from the base build up chamber (FIGS. 3-7 ) and HCl,which is removed from the acid build up chamber (FIGS. 3-7 ).Desalinated water may be optionally removed from the cell, although thisstream is not shown in FIG. 18 . Whether or not desalinated water isremoved from the cell depends on the concentration of the feed brine,i.e. the aqueous LiCl solution, as well as the desired concentration ofLiOH and HCl that are produced.

In an analogous manner, water may optionally be fed to the cell, ratherthan being removed. Whether or not water is fed to the cell depends onthe concentration of the feed brine, i.e. the aqueous LiCl solution, aswell as the desired concentration of LiOH and HCl that are produced. Asshown in FIG. 18 , in this embodiment, the HCl can be used to regeneratethe ion exchange resins that are used to remove Ca, Mg, Na, and K fromthe LiCl process stream that enters the cell. The HCl may also be usedin the boron removal step to regenerate the ion exchange resin, which istypically after the evaporation/precipitation step near the beginning ofthe process. The HCl may be used for pH adjustment of the process streamwhich generates CO₂ as shown in the FIG. 18 . The CO₂ can be combinedwith a portion of the LiOH product stream, thereby producing a streamthat comprises LiOH and Li₂CO₃. The LiOH/Li₂CO₃ stream can be fed to theprecipitation step that removes Ca and Mg, as shown in FIG. 18 .Importantly, not all of the LiOH product stream is used in thisprecipitation step, because the LiOH is the desired product. However,the ability to use the LiOH in this way, reduces significantly the needto buy a base such as NaOH or Na₂CO₃ to effect the precipitation removalof Ca and Mg.

As a point of reference, based on a test using 6% LiCl stream used as abrine feed, it takes about 150-250 kWh/m³ of LiCl brine to reduce thetotal salt content to 3% when air is used at the ODC.

FIG. 19 depicts a second embodiment implementation of the membraneelectrochemical cell in the recovery of lithium from a salar brine. Inthis embodiment, an aqueous LiCl solution is again the brine feed to themembrane electrolysis cell. In this embodiment, like the firstembodiment, the membrane electrolysis cell is used to convert LiCl intoLiOH. The aqueous solution of LiCl is fed to the salt depletion chamberof the membrane electrolysis cell, shown in any of FIGS. 3-7 in moredetail. Oxygen gas, which is preferably in the form of air, is fed tothe cathode. The oxygen or air may optionally be humidified, e.g., bybubbling the gas through water before feeding it to cathode and the airmay optionally be purified. As shown, the outputs from the cell areLiOH, which is removed from the base build up chamber (FIGS. 3-7 ) andHCl, which is removed from the acid build up chamber (FIGS. 3-7 ). As inthe first embodiment, desalinated water may be optionally removed fromthe cell, although this stream is not shown in FIG. 19 .

Whether or not desalinated water is removed from the cell depends on theconcentration of the feed brine, i.e. the aqueous LiCl solution, as wellas the desired concentration of LiOH and HCl that are produced. In ananalogous manner, water may optionally be fed to the cell, rather thanbeing removed. Whether or not water is fed to the cell depends on theconcentration of the feed brine, i.e. the aqueous LiCl solution, as wellas the desired concentration of LiOH and HCl that are produced. In thisembodiment, all of the LiOH that is produced is removed, i.e. there isnot a recycle stream comprising LiOH.

However, the HCl stream as in the first embodiment may be recycled andused in the lithium recovery process. As shown in FIG. 19 , the HCl isused to regenerate the ion exchange resins used to remove the Ca and Mgfrom the process stream just prior to the stream being fed to themembrane electrolysis cell as the feed brine. The HCl produced may alsobe used to in the removal of the boron B, after the precipitation stepto regenerate the ion exchange resin.

FIG. 20 shows a third embodiment illustrating use of the membraneelectrolysis cell in a lithium production process. In this embodiment ofthe process, a mixed brine solution comprising both LiCl and NaCl arefed to the membrane electrolysis cell. The cell then produces HCl and amixed LiOH and NaOH solution. This mixed LiOH and NaOH solution is fedto a crystallization/separation step that produces crystalized LiOH anda mixed solution of NaOH and a lower concentration of LiOH than themixed LiOH and NaOH solution that was fed to thecrystallization/separation step. As can be seen in FIG. 20 , the processis similar to that of FIGS. 16 and 18 , but utilizes the membraneelectrolysis cell into an existing operation to produce LiOH from salarbrine. In an alternative embodiment, the membrane electrolysis cell maybe applied to another waste or recycle lithium chloride stream producedin a conventional lithium operation that does not have sodium, and couldconvert the lithium chloride to lithium hydroxide and hydrochloric acid.

The steps are thus:

-   -   Step 1: The mixed lithium chloride and sodium chloride stream        that is produced from a conventional salar brine processing        operation is fed to the membrane electrolysis cell        (electrochemical cell) to produce a mixed lithium hydroxide and        sodium hydroxide solution.    -   Step 2: The mixed lithium hydroxide and sodium hydroxide are        sent to a crystallizer/separator where they are separated due to        the large solubility difference between the two salts—the NaOH        is much more soluble in water than the LiOH. The        crystallization/separation unit may either evaporate and        optionally re-condense the water, or may simply effect the        precipitation of some of the LiOH by cooling the mixed solution        of NaOH and LiOH. The more typical method is simply to evaporate        the water. The lithium hydroxide is crystallized while the        sodium hydroxide remains in solution. The crystallized lithium        hydroxide is ready for market.    -   Step 3: Some lithium hydroxide remains in solution with the        sodium hydroxide and is recycled back to the process for use in        the precipitation stages.    -   Step 4: Some lithium hydroxide and sodium hydroxide is combined        with carbon dioxide to produce a mixed lithium carbonate and        sodium carbonate stream which is recycled back to the overall        process and used for further precipitation, which can be seen in        FIG. 20 .

Taken together, these steps result in a closed or nearly closed loop forsodium hydroxide, sodium carbonate and lithium carbonate by theincorporation of the electrochemical cell (membrane electrolysis cell)into the overall process to recover LiOH from salar brine.

Turning next to FIG. 21 , fourth embodiment use of the membraneelectrolysis cell in a lithium production process is shown. As shown inFIG. 21 , the membrane electrolysis cell is used to convert Li₂SO₄ toLiOH in a process where lithium is produced from a lithium-containingore. However, a person having skill in the art can appreciate that thebrine feed stream which comprises an aqueous solution of Li₂SO₄ does notnecessarily have to be from a lithium ore-based process.

In certain brine recovery processes it is desirable to convert a Li₂SO₄solution to a LiOH, and so the membrane electrolysis cell could be usedin such a process as well. As shown in FIG. 21 , the membraneelectrolysis cell uses an aqueous solution of Li₂SO₄. Also fed to thecell, as in the other embodiments, is a gas stream which comprisesoxygen. This stream which preferably is air, is fed to the cathode. Theoxygen or air may optionally be humidified, e.g., by bubbling the gasthrough water before feeding it to cathode and the air may optionally bepurified. As shown, the outputs from the cell are LiOH, which is removedfrom the base build up chamber (FIGS. 3-7 ) and H₂SO₄, which is removedfrom the acid build up chamber (FIGS. 3-7 ).

As in the first embodiment and the second embodiment, desalinated watermay be optionally removed from the cell, although this stream is notshown in FIG. 21 . Whether or not desalinated water is removed from thecell depends on the concentration of the feed brine, i.e. the aqueousLi₂SO₄ solution, as well as the desired concentration of LiOH and H₂SO₄that are produced. In an analogous manner, water may optionally be fedto the cell, rather than being removed. Whether or not water is fed tothe cell depends on the concentration of the feed brine, i.e. theaqueous Li₂SO₄ solution, as well as the desired concentration of LiOHand H₂SO₄ that are produced.

In this embodiment, both the LiOH and the H₂SO₄ are recycled back intothe lithium recovery process, which mitigates at least some of the needto buy additional reagents. Importantly, only a portion of the LiOH isrecycled, since of course the LiOH is a desirable end product. The H₂SO₄is used in the acid roasting step of ore production, in order to producethe Li₂SO₄ brine solution after the water leaching step. A portion ofthe LiOH that is produced can be used to precipitate out Ca and Mg fromthe Li₂SO₄ brine solution after the water leaching step, as shown inFIG. 20 .

FIG. 22 shows an exemplary embodiment of the use of the membraneelectrolysis cell in a closed-loop process in which lithium carbonate(Li₂CO₃) produced from other methods, for example, lithium carbonateproduced from brine operations by precipitation with the use of sodiumcarbonate or lithium carbonate produced from jadarite (LiNaSiB₃O₇OH) maybe dissolved in hydrochloric acid to produce a lithium chloride solutionwhich is converted to LiOH. The process as shown in FIG. 22 proceeds asfollows:

-   -   Step 1: Lithium carbonate produced from other methods is        converted to lithium chloride by dissolution in hydrochloric        acid.    -   Step 2: The lithium chloride is processed through the        electrochemical cell to produce lithium hydroxide and        hydrochloric acid.    -   Step 3: The hydrochloric acid is recycled back for further        conversion of lithium carbonate to lithium chloride resulting in        a completely or substantially closed loop system.

As shown in the following three exemplary embodiments (FIGS. 23, 24, and25 ), the membrane electrolysis cell may also be used in lithiumrecovery processes that incorporate ion exchange resins. These ionexchange resins may be used either directly to produce LiOH, or they maybe used to recycle and/or recover other ionic species during the lithiumrecovery process. The use of the advantages of these embodiments (andall embodiments disclosed herein) are manifold vis-à-vis operational andcapital cost savings.

As discussed above, lithium hydroxide is produced by processing lithiumrich brines, such as salar brines, through an extensive process. Thewater in the brine is allowed to evaporate over a period of 6 to 18months to concentrate the lithium chloride in the solution to 5 wt. %LiCl or higher and to precipitate out significant sodium, calcium andmagnesium salt species, since these are in general less soluble than theLiCl.

The lithium chloride-rich brine must then be subjected to a variety ofpurification steps. These purification steps may include for example:boron removal through a solvent or other means, calcium and magnesiumremoval through the addition of lime (calcium oxide and/or calciumhydroxide) and caustic soda, soda ash and/or sodium bicarbonate or otherspecies, further calcium and magnesium removal through the addition ofsoda ash, i.e., sodium carbonate Na₂CO₃. These processes produce a mixedlithium chloride and sodium chloride stream to which additional soda ashis added resulting in the precipitation of lithium carbonate. Thelithium carbonate may then be crystallized. Currently, this crystallizedlithium carbonate is often transported to a lithium hydroxide plantwhere it is converted to lithium hydroxide by adding calcium hydroxide.The lithium hydroxide is then crystallized for sale. There are severalprocess units associated with all these steps and clearly, procuring andmaintaining these process units represents significant capitalinvestment as well as on-going operational costs.

An ion exchange resin selective to adsorption or binding of lithium canbe used to eliminate many of these steps to selectively adsorb lithiumfrom the salar brine (or other source) without the need fortime-consuming evaporation or removal of boron, calcium, magnesium, etc.For instance, an ion exchange resin for selectively binding lithium andproducing lithium chloride by desorbing the lithium from the resin withHCl could be utilized. A membrane electrolysis cell as disclosed hereincan convert the lithium chloride to lithium hydroxide and thehydrochloric acid, which would be recycled back to the ion exchangeresin, which the desirable lithium hydroxide is collected.

In another embodiment, an ion exchange resin for selective adsorption oflithium could also be used to produce lithium sulfate by regeneration ofthe resin with sulfuric acid. Analogously, the lithium sulfate would befed to the electrolysis cell to produce lithium hydroxide and sulfuricacid. The sulfuric acid would be recycled back to the ion exchange resinto produce more lithium sulfate, while the desirable lithium hydroxideis collected.

The ion exchange resin would eliminate significant capital and operatingexpenditures and costs associated with the lithium evaporation ponds andassociated downstream transportation.

Eliminating the evaporation ponds would also conserve water which islost to the atmosphere during the evaporation step. Producers would beable to pump the lithium-depleted brine back to the salar brinereservoir, which conserves the water which would be evaporated. Thisfeature is critical from both environmental and legal viewpoints. Chile,for example, where most of the world's lithium brines are located, hasstrict limits on water usage and the amount of brine lithium producerscan pump. The purpose of the regulations is to conserve the scarce waterin the Salar desert region of Chile. Therefore, these limits effectivelymean that the producers' production of lithium is limited. However, ifthe lithium-depleted brine from the ion exchange process is pumped backto the reservoirs, much less net brine is pumped, and the producers canincrease their production of lithium without exceeding the governmentallimits on the amount of salar brine they can pump or amount of waterused in the operation. Use of ion exchange resins in the lithiumrecovery process would also save time, since the evaporation step isslow. Additionally and importantly, the need to purchase reagentsnecessary for precipitation of calcium and magnesium would beeliminated.

The largest cost associated with direct production of lithium with ionexchange resins is with the need to procure HCl which is required todesorb or unbind the lithium ion from the active sites and regeneratethe ion-exchange resin. An electrochemical cell as described herein thatis capable of converting lithium chloride to lithium hydroxide andhydrochloric acid not only eliminates the need to procure the reagentrequired for the conversion of lithium chloride to lithium carbonate tolithium hydroxide, but these cells also produce the vital HCl requiredto extract the lithium from the ion exchange resin. Accordingly, thewhole process starting from lithium chloride evaporation to lithiumcarbonate production and lithium carbonate conversion through toproduction of lithium hydroxide for use in batteries may be simplifiedto the use of only an ion exchange resin and an electrochemical cell.

In addition, another exemplary use of the membrane electrolysis cells inprocesses utilizing ion exchange resins to directly adsorb lithium frombrine is a process where the ion exchange resin is deployed in thedesert where the salar brine is pumped, while the membrane electrolysiscell is in a different location. In this exemplary process the ionexchange resin is removed, transported to the location of the membraneelectrolysis cell where the ion exchange resin is regenerated with HClthus producing LiOH. The ion-exchange resin would then be shipped backto the brine site in the desert. Thus, a used ion-exchange resin wouldmove one way and the regenerated ion exchange resin would move theopposite way. Therefore, in any of the exemplary processes shown in FIG.22, 23 , or 24 below, the membrane electrolysis cell could be located ina different location from the ion exchange resin.

Non-limiting examples of suitable such ion exchange resins are thosethat selectively binds lithium or another precious metal based on the pHof the solution. For example, the resin may bind lithium in acid but notin alkali, or vice versa. This allows us to be able to regenerate theresin and extract lithium from it. This allows the producers to be ableto regenerate the resin and extract lithium from it. The membraneelectrolysis cell then produces the appropriate pH solution to removethe bound ion by providing HCl or NaOH. Such ion exchange resins mayalso include complexed metal resins such as H_(n)M_(n)O_(n) where the His hydrogen, M is a metal species, O is oxygen and n is an integer.Non-limiting examples include LiAlO₂, LiCuO₂, among others.

The following two embodiments thus demonstrate how the membraneelectrolysis cell as disclosed herein may be incorporated into lithiumrecovery processes in which an ion exchange resin is used to directlyproduce LiOH.

FIG. 23 shows an exemplary embodiment of a use of the membraneelectrolysis cell in a lithium production process where an ion exchangeresin is used to selectively adsorb Li from a lithium brine. Thislithium brine does not have to be a salar brine—it can be a brine fromother industrial processes such as produced water from oil and gasoperations, or geothermal brines which sometimes have lithium ornaturally occurring saline aquifers, or the brine can be derived from alithium ion battery recycling process. As can be seen in FIG. 23 , themembrane electrolysis cell simultaneously produces LiOH, which can besold, but also produces HCl which is used to remove the Li (as LiCl)from the ion exchange resin, thus regenerating the ion exchange resin.The LiCl is fed to the membrane electrolysis cell to produce the desiredLiOH. In an alternative embodiment, lithium sulfate could be produced byusing sulfuric acid and producing lithium sulfate which can be used toproduce the desired lithium hydroxide in the membrane electrolysis cell.The steps in the process are as follows:

-   -   Step 1: Lithium containing brine or solution is processed with        an ion exchange resin or other adsorbing agent to adsorb lithium        out of the brine or solution.    -   Step 2: The lithium containing resin bead or adsorbent is        regenerated with hydrochloric acid to produce a lithium chloride        solution. The resin or adsorbing agent is regenerated to the        proton form by HCl. Alternatively, the resin could be        regenerated with sulfuric acid.    -   Step 3: The lithium chloride solution is processed through the        electrochemical cell to produce lithium hydroxide and        hydrochloric acid. Alternatively, the lithium sulfate solution        could be processed through the electrochemical call to produce        lithium hydroxide and sulfuric acid.    -   Step 4: The lithium hydroxide is sold to market or otherwise        removed from the process while the hydrochloric acid is recycled        back to Step 2.

FIG. 24 shows another exemplary use of the membrane electrolysis cell asdisclosed herein in which the lithium-containing brine is subjected to aprocess in which boron is removed before the brine is sent to the ionexchange resin and then to the membrane electrolysis cell. As shown inFIG. 23 , in this exemplary embodiment, the process steps after theboron removal are:

-   -   Step 1: Lithium containing brine or solution is processed with        an ion exchange resin or other adsorbing agent to adsorb lithium        out of the brine or the solution.    -   Step 2: The lithium-containing ion exchange resin beads or other        suitable lithium adsorbent is regenerated with hydrochloric acid        to produce a lithium chloride solution. The resin or adsorbing        agent is regenerated to the proton form by HCl. Note that a        lithium depleted solution may be pumped back to the salar        reservoir or pond. As noted above, since there are typically        water conservation limits in place, particularly in Chile,        regarding the amount of salar brine that may be pumped out of        the natural reservoirs, if the depleted solution is sent back to        the reservoir, the producer may then produce more lithium        without exceeding their legal limit regarding the amount of        brine that they can pump.    -   Step 3: The lithium chloride solution is processed through the        membrane electrochemical cell disclosed herein to produce        lithium hydroxide and hydrochloric acid. Alternatively, lithium        sulfate is process through the membrane electrochemical cell to        produce lithium hydroxide and sulfuric acid.    -   Step 4: The lithium hydroxide is sold to market or otherwise        removed from the process while the hydrochloric acid or sulfuric        acid is recycled back to Step 2.

Note that in the process as shown in FIG. 24 , air and electricity arefed to the membrane electrolysis cell. The overall reactions at theanode and cathode of the electrolysis are therefore:

Anode:2H₂O→O₂+4H++4e−

Cathode:O₂+2H₂O+4e−→4OH−

FIG. 25 shows yet another exemplary embodiment process for use of themembrane electrolysis cell utilizing the inventive gas diffusionelectrode at the cathode of the cell. In this process, lithium carbonateand/or bicarbonate is directly converted in the membrane electrolysiscell to lithium hydroxide. In this case, the reactions are as follows:

Cathode:O₂+2H₂O+4e−→4OH−

Anode:2H₂O→4H++4e−+O₂

The lithium carbonate would react with the protons generated at theanode to liberate lithium ions, carbon dioxide and water as follows:

2Li₂CO₃+4H+→4Li++2CO₂+2H₂O

The liberated lithium ions would be transported towards the cathode tothe lithium hydroxide compartment where they would combine with thehydroxide ions produced at the cathode and produce a lithium hydroxidesolution.

It should be understood that in all of the foregoing Embodimentsdepicting the use of the inventive membrane electrolysis cell inrecovery processes for lithium, that the role of GDEs essentially remainthe same between the various applications: to produce OH− ions from ahumidified oxygen/air gas stream. Regardless of the salt used as thefeed brine (LiCl, NaCl, Na₂SO₄, Li₂SO₄), the cathode catalyst on the GDEalways plays the same role.

Various non-limiting aspects of the invention may be summarized asfollows:

-   -   Aspect 1: A process for recovering Li from a Li source, the        process comprising the steps of:

receiving, in a membrane electrolysis cell, a salt-containing solutionand a gas comprising O₂; and

delivering, from the membrane electrolysis cell, recovered Li and/orreagent materials used in the process for recovering Li.

-   -   Aspect 2: The process of Aspect 1, further comprising:

receiving, in the membrane electrolysis cell, byproducts of the processfor recovering Li; and

delivering, from the membrane electrolysis cell, reagent materials usedin the process for recovering Li.

-   -   Aspect 3: The process of Aspect 1 or Aspect 2, wherein the        membrane electrolysis cell comprises

an inlet through which the salt-containing solution is received into aninterior of the membrane electrolysis cell;

-   -   an anode positioned to extend within the interior of the        membrane electrolysis cell and positioned in an anode        compartment;    -   a cathode comprising a gas diffusion electrode positioned to        extend within the interior of the membrane electrolysis cell and        positioned in a cathode compartment, the gas diffusion electrode        including a diffusion layer configured to diffuse gas and a        hydrophilic catalyst layer disposed on a surface of the        diffusion layer, the hydrophilic catalyst layer having a        hydrophilicity greater than that of the diffusion layer and the        hydrophilic catalyst layer being configured to transport        negative ions;    -   a gas inlet positioned in the cathode compartment through which        the gas comprising O₂ is introduced into contact with the gas        diffusion electrode;    -   a first ion exchange membrane interposed between the anode        compartment and the hydrophilic catalyst layer of the gas        diffusion electrode, the first ion exchange membrane being        configured to exchange ions received from the anode to an        opposed surface of the first ion exchange membrane; and

at least one outlet through which the recovered Li and/or reagentmaterials used in the process for recovering Li is removed from aninterior of the membrane electrolysis cell;

wherein, in performing the process the salt-containing solution isreceived into the anode compartment and positive salt ions and negativesalt ions are formed from the salt-containing solution in the anodecompartment; and wherein the gas comprising O₂ is reduced at the cathodeto form OH−;

wherein in performing the process the positive salt ions move throughthe first ion exchange membrane to the opposed surface of the first ionexchange membrane; and

wherein the positive salt ions combine with the OH− to form therecovered Li and/or reagent materials used in the process for recoveringLi.

-   -   Aspect 4: The process of Aspect 3, the diffusion layer having a        bi-layer construction formed from a plurality of diffusion        sublayers, wherein the diffusion sublayers are hydrophobic and        wherein water is transported away from the diffusion sublayers.    -   Aspect 5: The process of Aspect 3, the gas diffusion electrode        further comprising a hydrophobic catalyst layer disposed on a        surface of the diffusion layer that is opposite from the        hydrophilic catalyst layer, the hydrophobic catalyst layer        having a hydrophilicity less than that of the diffusion layer        and being capable of transporting negative ions, wherein the OH−        ions are transported through the hydrophobic catalyst layer.    -   Aspect 6: The process of Aspect 5, the membrane electrolysis        cell further comprising:

a second ion exchange membrane, the second ion exchange membrane beingdisposed on the hydrophilic catalyst layer of the gas diffusionelectrode and being configured to exchange ions received from thehydrophilic catalyst layer of the gas diffusion electrode to an opposedsurface of the second ion exchange membrane;

wherein the first and second ion exchange membranes define a base buildup compartment interposed between the cathode compartment and the anodecompartment;

wherein the OH− ions are exchanged through the second ion exchangemembrane to the opposed surface of the second ion exchange membrane intothe base build up compartment;

wherein the OH− ions combine with the positive salt ions in the basebuild up compartment to form the recovered Li and/or reagent materialsused in the process for recovering Li; and

wherein the recovered Li and/or reagent materials used in the processfor recovering Li is removed from the base buildup compartment.

-   -   Aspect 7: The process of Aspect 6, the membrane electrolysis        cell further comprising:    -   a third ion exchange membrane, the third ion exchange membrane        being interposed between the first ion exchange membrane and the        anode compartment, wherein the first and third ion exchange        membranes define a salt depletion compartment interposed between        the anode compartment and the base build up compartment, the        third ion exchange membrane being configured to exchange ions        received from the salt depletion compartment to an opposed        surface of the third ion exchange membrane and into the anode        compartment;    -   wherein the salt containing solution is received into the salt        depletion compartment and the positive salt ions and the        negative salt ions are formed from the salt-containing solution        in the salt depletion compartment; and    -   wherein the negative salt ions are exchanged from the salt        depletion compartment to the opposed surface of the third ion        exchange membrane.    -   Aspect 8: The process of Aspect 7, the membrane electrolysis        cell further comprising:    -   a fourth ion exchange membrane, the fourth ion exchange membrane        being interposed between the third ion exchange membrane and the        anode compartment, wherein the third and the fourth ion exchange        membranes define an acid build up compartment interposed between        the anode compartment and the salt depletion compartment, the        fourth ion exchange membrane being configured to exchange ions        received from the anode compartment to an opposed surface of the        fourth ion exchange membrane and into the acid build up        compartment;    -   wherein H+ ions are formed in the anode compartment and the H+        ions are exchanged from the anode compartment to the opposed        surface of the fourth ion exchange membrane into the acid build        up compartment;    -   wherein the H+ ions and the negative salt ions together form an        acid, wherein the acid comprises the recovered Li and/or reagent        materials used in the process for recovering Li; and    -   wherein the acid is removed from the acid build up compartment.    -   Aspect 9: The process of Aspect 8, wherein the first and fourth        ion exchange membranes comprise cation exchange membranes and        the second and third ion exchange membranes comprise anion        exchange membranes.    -   Aspect 10: The process of any of Aspects 1-9, wherein the gas        comprising O₂ is air.    -   Aspect 11: The process of any of Aspects 1-9, wherein the gas        comprising oxygen is a waste stream from a nitrogen producing        operation.    -   Aspect 12: The process of any of Aspects 1-11, wherein the Li        source comprises a salar brine and the recovered Li comprises at        least one of LiOH, Li₂CO₃, and/or LiCl.    -   Aspect 13: The process of any of Aspects 1-12, wherein in the        receiving step the byproducts of the process for recovering Li        comprise NaCl precipitated from the salar brine; and in the        delivering step the reagent materials used in the process for        recovering Li comprise HCl and NaOH.    -   Aspect 14: The process of any of Aspects 1-13 wherein the        reagent materials are used to regenerate and/or desorb ions from        an ion exchange resin used in the process for recovering        lithium.    -   Aspect 15: The process of Aspect 13 wherein at least one of the        HCl and the NaOH are used to regenerate and/or desorb ions from        an ion exchange resin used in the process for recovering        lithium.    -   Aspect 16: The process of any of Aspects 1-14, wherein the        lithium source comprises lithium ore and the recovered lithium        comprises at least one of LiOH, Li₂CO₃, and/or Li₂SO₄.    -   Aspect 17: The process of Aspect 16, wherein the recovered        lithium comprises LiOH and the process further comprises a step        of recycling a portion of the LiOH into the process for        recovering lithium.    -   Aspect 18: The process of Aspect 17, further comprising a step        of precipitating at least one of calcium and magnesium with the        portion of the LiOH.    -   Aspect 19: The process of Aspect 17, further comprising a step        of reacting the portion of the LiOH with CO₂ to produce Li₂CO₃,        and further comprising a step of precipitating at least one of        calcium and magnesium with the Li₂CO₃.    -   Aspect 20: The process of any of Aspects 1-16, wherein in the        receiving step the byproducts of the process for recovering Li        comprise Na₂SO₄ and in the delivering step the reagent materials        used in the process for recovering lithium comprise H₂SO₄ and        NaOH.    -   Aspect 21: The process of any of Aspects 1-14, wherein the        lithium source comprises a salar brine and in the receiving step        the salt-containing solution comprises LiCl and in the        delivering step the recovered lithium comprises LiOH and the        reagent materials used in the lithium recovery process comprise        HCl.    -   Aspect 22: The process of Aspect 21, further comprising        recycling a portion of the LiOH into the process for recovering        lithium.    -   Aspect 23: The process of either of Aspect 21 or Aspect 22,        further comprising a step of reacting the portion of the LiOH        with CO₂ to produce Li₂CO₃, and further comprising a step of        precipitating at least one of calcium and magnesium with the        Li₂CO₃.    -   Aspect 24: The process of any of Aspects 1-14, wherein the        lithium source comprises a salar brine and in the receiving step        the salt-containing solution comprises LiCl and NaCl and in the        delivering step the recovered lithium comprises LiOH and the        reagent materials used in the lithium recovery process comprise        HCl and NaOH.    -   Aspect 25: The process of Aspect 20, further comprising        recycling a portion of the LiOH into the lithium recovery        process.    -   Aspect 26: The process of any of Aspects 1-16, wherein the        lithium source comprises lithium ore and in the receiving step        the salt-containing solution comprises Li₂SO₄ from the lithium        ore and in the delivering step the recovered lithium comprises        LiOH and the reagent materials used in the lithium recovery        process comprise H₂SO₄.    -   Aspect 27: The process of Aspect 18, wherein the salt-containing        solution comprising LiCl is derived by contacting salar brine        with an ion exchange resin, wherein the ion exchange resin is        configured to adsorb lithium from the salar brine and to desorb        the adsorbed lithium in the form of the solution comprising the        LiCl in the presence of the HCl.    -   Aspect 28: The process of Aspect 26, wherein the salt-containing        solution comprising Li₂SO₄ is derived by contacting a lithium        containing-stream derived from the lithium ore with an ion        exchange resin, wherein the ion exchange resin is configured to        adsorb lithium from the lithium containing-stream derived from        the lithium ore and to desorb the adsorbed lithium in the form        of the solution comprising the Li₂SO₄ in the presence of the        H₂SO₄.    -   Aspect 29: The process of Aspect 22, wherein the ion exchange        resin is further configured to adsorb lithium directly from the        salar brine.    -   Aspect 30: The process of Aspect 22, wherein the process further        comprises a step of removal of boron from the salar brine prior        to contacting the salar brine with the ion exchange resin.    -   Aspect 31: The process of any of Aspects 1-16, wherein the        lithium source comprises Li₂CO₃.    -   Aspect 32: The process of Aspect 31, wherein in the receiving        step the salt-containing solution comprises LiCl or Li₂SO₄.    -   Aspect 33: The process of Aspect 32, wherein in the delivering        step the recovered lithium comprises LiOH and the reagent        materials used in the lithium recovery process comprise HCl.    -   Aspect 34: The process of Aspect 32, wherein in the delivering        step the recovered lithium comprises LiOH and the reagent        materials used in the lithium recovery process comprise H₂SO₄.    -   Aspect 35: The process of Aspect 33, wherein the process further        comprises recycling the HCl to dissolve the Li₂CO₃ to produce        the LiCl.    -   Aspect 36: The process of Aspect 34, wherein the process further        comprises recycling the H₂SO₄ to dissolve the Li₂CO₃ to produce        the Li₂SO₄.    -   Aspect 37: The process of any of Aspects 1-11, wherein the        lithium source comprises a brine derived from a lithium ion        battery recovery process.    -   Aspect 38: A gas diffusion electrode for use in a membrane        electrolysis cell, the gas diffusion electrode comprising:    -   a diffusion layer configured to diffuse a gas;    -   a hydrophilic catalyst layer disposed on a surface of the        diffusion layer, the hydrophilic catalyst layer having a        hydrophilicity greater than that of the diffusion layer and        being capable of transporting negative ions; and    -   an ion exchange membrane disposed on a surface of the        hydrophilic catalyst layer, the ion exchange membrane being        configured to exchange ions from the hydrophilic catalyst layer        to an opposed surface of the ion exchange membrane.    -   Aspect 39: The gas diffusion electrode of Aspect 38, the        diffusion layer having a bi-layer construction formed from a        plurality of diffusion sublayers.    -   Aspect 40: The gas diffusion electrode of either of Aspect 38 or        Aspect 39, further comprising a hydrophobic catalyst layer        disposed on a surface of the diffusion layer that is opposite        from the hydrophilic catalyst layer, the hydrophobic catalyst        layer having a hydrophilicity less than that of the diffusion        layer and being capable of transporting negative ions.    -   Aspect 41: The gas diffusion electrode of any of Aspects 38-40,        the hydrophilic catalyst layer comprising platinum and carbon        and an anion exchange ionomer.    -   Aspect 42: The gas diffusion electrode of either of Aspect 40 or        Aspect 41, the hydrophobic catalyst layer comprising platinum        and carbon and an anion exchange ionomer.    -   Aspect 43: The gas diffusion electrode of any of Aspects 40-42,        the hydrophobic catalyst layer comprising PTFE.    -   Aspect 44: The gas diffusion electrode of any of Aspects 40-43,        at least one of the hydrophilic catalyst layer and the        hydrophobic catalyst layer being configured as an oxygen        depolarized cathode to catalyze the following reaction:        O₂+2H₂O+4e−→4OH−.    -   Aspect 45: The gas diffusion electrode of any of Aspects 38-44,        said ion exchange membrane being an anion exchange membrane,        thereby forming a cathode.    -   Aspect 46: A method of producing a gas diffusion electrode for        use in a membrane electrolysis cell, the method comprising:    -   disposing a hydrophilic catalyst layer on a surface of a        diffusion layer, the hydrophilic catalyst layer having a        hydrophilicity greater than that of the diffusion layer; and    -   disposing an ion exchange membrane on a surface of the catalyst        layer, the ion exchange membrane being configured to exchange        ions from the catalyst layer to an opposed surface of the ion        exchange membrane and to reduce or prevent flooding of the        catalyst layer.    -   Aspect 47: The method of Aspect 46, further comprising disposing        a hydrophobic catalyst layer on a surface of the diffusion layer        that is opposite from the hydrophilic catalyst layer, the        hydrophobic catalyst layer having a hydrophilicity less than        that of the diffusion layer.    -   Aspect 48: The method of either of Aspect 46 or Aspect 47, the        diffusion layer having a bilayer construction formed from a        plurality of diffusion sublayers, the hydrophilic catalyst layer        disposing step including disposing the hydrophilic catalyst        layer on a surface of one of the diffusion sublayers, and the        hydrophobic catalyst layer disposing step including disposing        the hydrophobic catalyst layer on an opposite surface of another        one of the diffusion sublayers.    -   Aspect 49: The method of any of Aspects 46-48, further        comprising combining the diffusion sublayers to form the        diffusion layer.    -   Aspect 50: The method of any of Aspects 46-49, at least one of        the hydrophilic catalyst layer and the hydrophobic catalyst        layer being formed from an ink and at least one of the        hydrophilic catalyst layer disposing step and the hydrophobic        catalyst layer disposing step comprising applying the ink on the        surface of the diffusion layer.    -   Aspect 51: A membrane electrolysis cell for processing a        salt-containing solution, the membrane electrolysis cell        comprising:    -   an inlet through which the salt-containing solution is        introduced into an interior of the membrane electrolysis cell;    -   an anode positioned to extend within the interior of the        membrane electrolysis cell and positioned in an anode        compartment;    -   a cathode comprising a gas diffusion electrode positioned to        extend within the interior of the membrane electrolysis cell and        positioned in a cathode compartment, the gas diffusion electrode        including a diffusion layer configured to diffuse gas and a        hydrophilic catalyst layer disposed on a surface of the        diffusion layer, the hydrophilic catalyst layer having a        hydrophilicity greater than that of the diffusion layer and the        hydrophilic catalyst layer being configured to transport        negative ions;    -   a gas inlet through which a gas comprising O₂ is introduced into        contact with the gas diffusion electrode;    -   a first ion exchange membrane interposed between the anode        compartment and the hydrophilic catalyst layer of the gas        diffusion electrode, the first ion exchange membrane being        configured to exchange ions received from the anode to an        opposed surface of the first ion exchange membrane; and    -   at least one outlet through which a product of the salt solution        is removed from an interior of the membrane electrolysis cell.    -   Aspect 52: The membrane electrolysis cell of Aspect 51, further        comprising    -   a second ion exchange membrane, the second ion exchange membrane        being disposed on the hydrophilic catalyst layer of the gas        diffusion electrode and being configured to exchange ions        received from the hydrophilic catalyst layer of the gas        diffusion electrode to an opposed surface of the third ion        exchange membrane;    -   wherein the first and second ion exchange membranes define a        base build up compartment interposed between the cathode        compartment and the anode compartment.    -   Aspect 53: The membrane electrolysis cell of Aspect 52, further        comprising    -   a third ion exchange membrane, the third ion exchange membrane        being interposed between the first ion exchange membrane and the        anode compartment, wherein the first and third ion exchange        membranes define a salt depletion compartment interposed between        the anode compartment and the base build up compartment, the        third ion exchange membrane being configured to exchange ions        received from the salt depletion compartment to an opposed        surface of the third ion exchange membrane and into the anode        compartment.    -   Aspect 54: The membrane electrolysis cell of Aspect 53, further        comprising:    -   a fourth ion exchange membrane, the fourth ion exchange membrane        being interposed between the third ion exchange membrane and the        anode compartment, wherein the third and the fourth ion exchange        membranes define an acid build up compartment interposed between        the anode compartment and the salt depletion compartment, the        fourth ion exchange membrane being configured to exchange ions        received from the anode compartment to an opposed surface of the        fourth ion exchange membrane and into the acid build up        compartment.    -   Aspect 55: The membrane electrolysis cell of Aspect 54 wherein        the first and fourth ion exchange membranes comprise cation        exchange membranes and the second and third ion exchange        membranes comprise anion exchange membranes.    -   Aspect 56: The membrane electrolysis cell of any of Aspects        51-55 wherein the hydrophilic catalyst layer comprises platinum        and carbon and an anion exchange ionomer.    -   Aspect 57: A process for purifying or concentrating LiOH using a        membrane electrolysis cell, the steps of the process comprising        the steps of:    -   receiving, in a membrane electrolysis cell, a feed solution        comprising LiOH and a gas comprising O₂; and    -   delivering, from the membrane electrolysis cell, a product        solution comprising a purified LiOH solution and/or a        concentrated LiOH solution.    -   Aspect 58: A process for producing LiOH using a membrane        electrolysis cell, the steps of the process comprising the steps        of:    -   receiving, in a membrane electrolysis cell, a feed solution        comprising Li₂CO₃ and a gas comprising O₂; and    -   delivering, from the membrane electrolysis cell, a product        solution comprising a purified LiOH solution and/or a        concentrated LiOH solution.

EXAMPLES

Experiments were conducted to demonstrate the effect of the inventivebilayer gas diffusion electrodes used as the oxygen depolarized cathodescompared to the single layer ODC.

FIG. 26 shows a current vs. time plot for a membrane electrolysis cellutilizing a single layer ODC. As can be seen in the plot, a single layerGDE using O₂ at the cathode after about 11 hours of testing starts toproduce H₂ which is an indication of the ineffectiveness of the GDE topromote the reaction: O₂+2H₂O+4e−→4OH−.

FIGS. 27 and 28 , respectively, demonstrate the effectiveness of thebilayer ODC's used with either O₂ at the cathode or air as the oxygensource at the cathode. Using either O₂ or air, it can be seen thathydrogen is not evolved, even after more than 30 hours of continuousoperation.

In some embodiments, the invention herein can be construed as excludingany element or process that does not materially affect the basic andnovel characteristics of the composition or process. Additionally, insome embodiments, the invention can be construed as excluding anyelement or process not specified herein.

As noted previously, although the invention is illustrated and describedherein with reference to specific embodiments, the invention is notintended to be limited to the details shown. Rather, variousmodifications may be made in the details within the scope and range ofequivalents of the claims and without departing from the invention.

Within this specification, embodiments have been described in a waywhich enables a clear and concise specification to be written, but it isintended and will be appreciated that embodiments may be variouslycombined or separated without departing from the invention. For example,it will be appreciated that all preferred features described herein areapplicable to all aspects of the invention described herein.

While preferred embodiments of the invention have been shown anddescribed herein, it will be understood that such embodiments areprovided by way of example only. Numerous variations, changes andsubstitutions will occur to those skilled in the art without departingfrom the spirit of the invention. Accordingly, it is intended that theappended claims cover all such variations as fall within the spirit andscope of the invention.

What is claimed is:
 1. A process for producing a base product, theprocess comprising the steps of: receiving, in a membrane electrolysiscell, a salt-containing solution comprising positive salt ions andnegative salt ions, and a gas comprising O₂; and delivering the baseproduct from the membrane electrolysis cell, wherein the membraneelectrolysis cell comprises: an anode positioned to extend within theinterior of the membrane electrolysis cell and positioned in an anodecompartment; a cathode comprising a gas diffusion electrode positionedto extend within the interior of the membrane electrolysis cell andpositioned in a cathode compartment, the gas diffusion electrodecomprising a first diffusion layer configured to diffuse gas and a firstcatalyst layer disposed on a surface of the first diffusion layer, thefirst catalyst layer having a hydrophilicity greater than that of thefirst diffusion layer and the first catalyst layer being configured totransport negative ions; a base build up compartment and a saltdepletion compartment interposed between the cathode compartment and theanode compartment, the base build up compartment is interposed betweenthe cathode compartment and the salt depletion compartment, and the saltdepletion compartment is interposed between the base build upcompartment and the anode compartment; a first anion exchange membrane,the first anion exchange membrane being disposed on the first catalystlayer of the gas diffusion electrode and being configured to exchangeions received from the first catalyst layer of the gas diffusionelectrode to an opposed surface of the first anion ion exchange membraneinto the base build up compartment; a cation exchange membraneinterposed between the salt depletion compartment and the base build upcompartment, the cation exchange membrane being configured to exchangeions received from the salt depletion compartment to an opposed surfaceof the cation exchange membrane into the base build up compartment; asecond anion exchange membrane interposed between the salt depletioncompartment and the anode compartment, the second anion exchangemembrane being configured to exchange ions received from the saltdepletion compartment to an opposed surface of the second anion exchangemembrane into the anode compartment; an inlet through which thesalt-containing solution is received into the salt depletioncompartment; a gas inlet positioned in the cathode compartment throughwhich the gas comprising O₂ is introduced into contact with the gasdiffusion electrode; and an outlet through which the base product isremoved from the base buildup compartment; wherein in performing theprocess: the salt-containing solution is received into the saltdepletion compartment; the positive salt ions migrate through the cationexchange membrane to the opposed surface of the cation exchange membraneinto the base build up compartment; the gas comprising O₂ is reduced atthe cathode to form OH−; the OH− ions migrate through the first anionexchange membrane to the opposed surface of the first anion exchangemembrane into the base build up compartment; the OH− ions combine withthe positive salt ions in the base build up compartment to produce thebase product; and the base product is removed from the base build upcompartment.
 2. The process of claim 1, wherein the gas diffusionelectrode further comprises a hydrophobic catalyst layer disposed on theopposite side of the first diffusion layer from the first catalystlayer.
 3. The process of claim 1, wherein the gas diffusion electrodefurther comprises a second diffusion layer disposed on the opposite sideof the first diffusion layer from the first catalyst layer.
 4. Theprocess of claim 3, wherein the gas diffusion electrode furthercomprises a hydrophobic catalyst layer disposed on the second diffusionlayer.
 5. The process of claim 1, wherein the first diffusion layer ishydrophobic.
 6. The process of claim 5, wherein the first catalyst layeris hydrophobic or hydrophilic.
 7. The process of claim 3, wherein thesecond diffusion layer is hydrophobic.
 8. The process of claim 1,wherein the base product comprises NaOH, KOH, or LiOH, or a mixturethereof.
 9. The process of claim 1, wherein the salt-containing solutioncomprises NaCl, Na₂SO₄, LiCl, Li₂SO₄, Li₂CO₃, or LiHCO₃.
 10. The processof claim 1, wherein the gas comprising oxygen is air, humidified air, ora waste stream from a nitrogen producing operation.
 11. A process forproducing a base product, the process comprising the steps of:receiving, in a membrane electrolysis cell, a salt-containing solutioncomprising positive salt ions and negative salt ions, and a gascomprising O₂; and delivering the base product from the membraneelectrolysis cell, wherein the membrane electrolysis cell comprises: ananode positioned to extend within the interior of the membraneelectrolysis cell and positioned in an anode compartment; a cathodecomprising a gas diffusion electrode positioned to extend within theinterior of the membrane electrolysis cell and positioned in a cathodecompartment, the gas diffusion electrode comprising a first diffusionlayer configured to diffuse gas and a first catalyst layer disposed on asurface of the diffusion layer, the first catalyst layer having ahydrophilicity greater than that of the first diffusion layer and thefirst catalyst layer being configured to transport negative ions; a basebuild up compartment, a salt depletion compartment and an acid build upcompartment interposed between the cathode compartment and the anodecompartment, the base build up compartment is interposed between thecathode compartment and the salt depletion compartment, the saltdepletion compartment is interposed between the base build upcompartment and the acid build up compartment, and the acid build upcompartment is interposed between the salt depletion compartment and theanode compartment; a first anion exchange membrane, the first anionexchange membrane being disposed on the first catalyst layer of the gasdiffusion electrode and being configured to exchange ions received fromthe first catalyst layer of the gas diffusion electrode to an opposedsurface of the first anion ion exchange membrane into the base build upcompartment; a first cation exchange membrane interposed between thesalt depletion compartment and the base build up compartment, the firstcation exchange membrane being configured to exchange ions received fromthe salt depletion compartment to an opposed surface of the first cationexchange membrane into the base build up compartment; a second anionexchange membrane interposed between the salt depletion compartment andthe acid build up compartment, the second anion exchange membrane beingconfigured to exchange ions received from the salt depletion compartmentto an opposed surface of the second anion exchange membrane into theacid build up compartment; a second cation exchange membrane interposedbetween the acid build up compartment and the anode compartment, thesecond cation exchange membrane being configured to exchange ionsreceived from the anode compartment to an opposed surface of the secondcation exchange membrane into the acid build up compartment; an inletthrough which the salt-containing solution is received into the saltdepletion compartment; a gas inlet positioned in the cathode compartmentthrough which the gas comprising O₂ is introduced into contact with thegas diffusion electrode; and an outlet through which the base product isremoved from the base buildup compartment; wherein in performing theprocess: the salt-containing solution is received into the saltdepletion compartment; the positive salt ions migrate through the firstcation exchange membrane to the opposed surface of the first cationexchange membrane into the base build up compartment; the gas comprisingO₂ is reduced at the cathode to form OH−; the OH− ions migrate throughthe first anion exchange membrane to the opposed surface of the firstanion exchange membrane into the base build up compartment; the OH− ionscombine with the positive salt ions in the base build up compartment toproduce the base product; and the base product is removed from the basebuild up compartment.
 12. The process of claim 11, wherein the gasdiffusion electrode further comprises a hydrophobic catalyst layerdisposed on the opposite side of the first diffusion layer from thefirst catalyst layer.
 13. The process of claim 11, wherein the gasdiffusion electrode further comprises a second diffusion layer disposedon the opposite side of the first diffusion layer from the firstcatalyst layer.
 14. The process of claim 13, wherein the gas diffusionelectrode further comprises a hydrophobic catalyst layer disposed on thesecond diffusion layer.
 15. The process of claim 11, wherein the firstdiffusion layer is hydrophobic.
 16. The process of claim 15, wherein thefirst catalyst layer is hydrophobic or hydrophilic.
 17. The process ofclaim 13, wherein the second diffusion layer is hydrophobic.
 18. Theprocess of claim 11, wherein the negative salt ions migrate through thesecond anion exchange membrane to the opposed surface of the secondanion exchange membrane into the acid build up compartment, H⁺ ions areformed in the anode compartment and migrate through the second cationexchange membrane into the acid build up compartment, the H⁺ ions andthe negative ions together form an acid in the acid build upcompartment, and the acid is removed from the acid build up compartment.19. The process of claim 18, wherein the acid comprises HCl or H₂SO₄.20. The process of claim 11, wherein the base product comprises NaOH,KOH, or LiOH, or a mixture thereof.
 21. The process of claim 11, whereinthe salt-containing solution comprises NaCl, Na₂SO₄, LiCl, Li₂SO₄,Li₂CO₃, or LiHCO₃.
 22. The process of claim 11, wherein the gascomprising oxygen is air, humidified air, or a waste stream from anitrogen producing operation.